diff --git a/backend/app/api/v1/routes_jobs.py b/backend/app/api/v1/routes_jobs.py
index 66c1fb4..7f94f40 100644
--- a/backend/app/api/v1/routes_jobs.py
+++ b/backend/app/api/v1/routes_jobs.py
@@ -8,7 +8,13 @@ from motor.motor_asyncio import AsyncIOMotorDatabase
 
 from ...core.config import settings
 from ...core.database import get_database
-from ...core.dependencies import get_accessible_project_ids, get_current_user, require_roles
+from ...core.dependencies import (
+    assert_job_in_user_org,
+    get_accessible_project_ids,
+    get_current_user,
+    get_user_org_ids,
+    require_roles,
+)
 from ...core.logging import get_logger
 from ...lib.vtt import VTTEditor
 from ...models.job import JobStatus, RequestedOutputs
@@ -75,6 +81,7 @@ async def create_job(
     file: UploadFile = File(...),
     brand_context: Optional[str] = Form(None),
     project_id: Optional[str] = Form(None),
+    client_id: Optional[str] = Form(None),
     request: Request = None,
     current_user: User = Depends(get_current_user),
     db: AsyncIOMotorDatabase = Depends(get_database),
@@ -103,11 +110,21 @@ async def create_job(
         file, f"{job_id}/source.mp4"
     )
 
+    # Resolve organization_id: prefer project's client_id, then form-supplied client_id
+    organization_id: Optional[str] = None
+    if project_id:
+        project_doc = await db.projects.find_one({"_id": project_id}, {"client_id": 1})
+        if project_doc:
+            organization_id = str(project_doc["client_id"])
+    if not organization_id and client_id:
+        organization_id = client_id
+
     # Create job document
     # Language is always "auto" - will be detected by AI during processing
     job_data = {
         "_id": job_id,
-        "client_id": str(current_user.id),
+        "client_id": organization_id or str(current_user.id),
+        "organization_id": organization_id,
         "title": title,
         "source": {
             "filename": f"{job_id}/source.mp4",
@@ -145,25 +162,6 @@ async def create_job(
         logger.info(f"Task state: {task.state}")
         logger.info(f"Task backend: {task.backend}")
 
-        # Try to get the task result to see if it was actually queued
-        try:
-            # This should timeout quickly since task just started
-            import time
-            time.sleep(1)  # Give it a moment
-            task_info = task.state
-            logger.info(f"Task state after 1 second: {task_info}")
-
-            # Check celery inspect to see active/scheduled tasks
-            from celery import current_app
-            i = current_app.control.inspect()
-            active_tasks = i.active()
-            scheduled_tasks = i.scheduled()
-            logger.info(f"Active tasks across all workers: {active_tasks}")
-            logger.info(f"Scheduled tasks across all workers: {scheduled_tasks}")
-
-        except Exception as e:
-            logger.warning(f"Could not inspect task status: {e}")
-
         # Store task ID in job document for monitoring
         await db.jobs.update_one(
             {"_id": job_id},
@@ -180,6 +178,7 @@ async def create_job(
     await log_job_action(
         AuditAction.JOB_CREATE, job_id, current_user, request,
         details={"title": title, "filename": file.filename},
+        organization_id=organization_id,
     )
     return JobResponse(
         id=job_id,
@@ -211,6 +210,8 @@ async def bulk_delete_jobs(
     already_deleted = 0
     errors = []
 
+    user_org_ids = await get_user_org_ids(current_user, db)
+
     for job_id in unique_job_ids:
         try:
             job_doc = await db.jobs.find_one({"_id": job_id})
@@ -220,6 +221,15 @@ async def bulk_delete_jobs(
                 logger.debug(f"Job {job_id} not found (may have been deleted by concurrent request)")
                 continue
 
+            # Tenant isolation: skip jobs outside the user's org
+            if user_org_ids is not None:
+                job_org = job_doc.get("organization_id") or job_doc.get("client_id")
+                if job_org not in user_org_ids:
+                    logger.warning(
+                        f"User {current_user.id} attempted cross-tenant bulk-delete on job {job_id}"
+                    )
+                    continue
+
             # Cancel task if exists
             task_id = job_doc.get("task_id")
             if task_id:
@@ -271,6 +281,7 @@ async def bulk_approve_jobs(
 
     approved_count = 0
     errors = []
+    user_org_ids = await get_user_org_ids(current_user, db)
 
     for job_id in unique_job_ids:
         try:
@@ -280,6 +291,16 @@ async def bulk_approve_jobs(
                 errors.append(f"Job {job_id}: not found")
                 continue
 
+            # Tenant isolation
+            if user_org_ids is not None:
+                job_org = job_doc.get("organization_id") or job_doc.get("client_id")
+                if job_org not in user_org_ids:
+                    logger.warning(
+                        f"User {current_user.id} attempted cross-tenant bulk-approve on job {job_id}"
+                    )
+                    errors.append(f"Job {job_id}: access denied")
+                    continue
+
             if job_doc["status"] != JobStatus.PENDING_QC.value:
                 errors.append(f"Job {job_id}: not in pending QC status")
                 continue
@@ -353,6 +374,7 @@ async def bulk_return_to_qc(
 
     returned_count = 0
     errors = []
+    user_org_ids = await get_user_org_ids(current_user, db)
 
     for job_id in unique_job_ids:
         try:
@@ -361,6 +383,16 @@ async def bulk_return_to_qc(
                 errors.append(f"Job {job_id}: not found")
                 continue
 
+            # Tenant isolation
+            if user_org_ids is not None:
+                job_org = job_doc.get("organization_id") or job_doc.get("client_id")
+                if job_org not in user_org_ids:
+                    logger.warning(
+                        f"User {current_user.id} attempted cross-tenant bulk-return-to-qc on job {job_id}"
+                    )
+                    errors.append(f"Job {job_id}: access denied")
+                    continue
+
             if job_doc["status"] not in RETURN_TO_QC_ELIGIBLE_STATUSES:
                 errors.append(f"Job {job_id}: cannot return to QC from status '{job_doc['status']}'")
                 continue
@@ -613,14 +645,8 @@ async def get_job(
             detail="Job not found"
         )
 
-    # Check access permissions
-    if current_user.role in (UserRole.CLIENT, UserRole.PROJECT_MANAGER):
-        accessible = await get_accessible_project_ids(current_user, db)
-        job_project_id = job_doc.get("project_id")
-        is_own_legacy = job_doc.get("client_id") == str(current_user.id) and not job_project_id
-        in_project = accessible is None or (job_project_id and job_project_id in (accessible or []))
-        if not is_own_legacy and not in_project:
-            raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Access denied")
+    # Enforce tenant isolation for every non-admin role
+    await assert_job_in_user_org(job_doc, current_user, db)
 
     # Check task status if task_id exists
     task_id = job_doc.get("task_id")
diff --git a/backend/app/api/v1/routes_review_notes.py b/backend/app/api/v1/routes_review_notes.py
index 9618f77..f7b2442 100644
--- a/backend/app/api/v1/routes_review_notes.py
+++ b/backend/app/api/v1/routes_review_notes.py
@@ -8,7 +8,7 @@ from fastapi import APIRouter, Depends, HTTPException, Query, status
 from motor.motor_asyncio import AsyncIOMotorDatabase
 
 from ...core.database import get_database
-from ...core.dependencies import get_current_user, require_roles
+from ...core.dependencies import assert_job_in_user_org, get_current_user, require_roles
 from ...core.logging import get_logger
 from ...models.user import User, UserRole
 from ...schemas.review_note import (
@@ -26,17 +26,18 @@ router = APIRouter(prefix="/jobs/{job_id}/review-notes", tags=["review-notes"])
 async def list_review_notes(
     job_id: str,
     asset_key: Optional[str] = Query(None, description="Filter notes by asset key"),
-    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.ADMIN)),
+    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.PROJECT_MANAGER, UserRole.ADMIN)),
     db: AsyncIOMotorDatabase = Depends(get_database),
 ):
     """List all review notes for a job, optionally filtered by asset key."""
-    # Verify job exists
+    # Verify job exists and enforce tenant isolation
     job = await db.jobs.find_one({"_id": job_id})
     if not job:
         raise HTTPException(
             status_code=status.HTTP_404_NOT_FOUND,
             detail="Job not found"
         )
+    await assert_job_in_user_org(job, current_user, db)
 
     # Build query
     query = {"job_id": job_id}
@@ -57,24 +58,27 @@ async def list_review_notes(
 async def create_review_note(
     job_id: str,
     request: ReviewNoteCreateRequest,
-    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.ADMIN)),
+    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.PROJECT_MANAGER, UserRole.ADMIN)),
     db: AsyncIOMotorDatabase = Depends(get_database),
 ):
     """Create a new review note for a video asset."""
-    # Verify job exists
+    # Verify job exists and enforce tenant isolation
     job = await db.jobs.find_one({"_id": job_id})
     if not job:
         raise HTTPException(
             status_code=status.HTTP_404_NOT_FOUND,
             detail="Job not found"
         )
+    await assert_job_in_user_org(job, current_user, db)
 
     # Create note document
     note_id = str(ObjectId())
+    organization_id = job.get("organization_id")
     now = datetime.utcnow()
 
     note_data = {
         "_id": note_id,
+        "organization_id": organization_id,
         "job_id": job_id,
         "asset_key": request.asset_key,
         "timestamp_seconds": request.timestamp_seconds,
@@ -95,10 +99,14 @@ async def create_review_note(
 async def get_review_note(
     job_id: str,
     note_id: str,
-    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.ADMIN)),
+    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.PROJECT_MANAGER, UserRole.ADMIN)),
     db: AsyncIOMotorDatabase = Depends(get_database),
 ):
     """Get a single review note by ID."""
+    job = await db.jobs.find_one({"_id": job_id}, {"_id": 1, "organization_id": 1, "project_id": 1, "client_id": 1})
+    if not job:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Job not found")
+    await assert_job_in_user_org(job, current_user, db)
     note = await db.review_notes.find_one({"_id": note_id, "job_id": job_id})
     if not note:
         raise HTTPException(
@@ -114,10 +122,14 @@ async def update_review_note(
     job_id: str,
     note_id: str,
     request: ReviewNoteUpdateRequest,
-    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.ADMIN)),
+    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.PROJECT_MANAGER, UserRole.ADMIN)),
     db: AsyncIOMotorDatabase = Depends(get_database),
 ):
     """Update a review note. Only the note owner can update."""
+    job = await db.jobs.find_one({"_id": job_id}, {"_id": 1, "organization_id": 1, "project_id": 1, "client_id": 1})
+    if not job:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Job not found")
+    await assert_job_in_user_org(job, current_user, db)
     note = await db.review_notes.find_one({"_id": note_id, "job_id": job_id})
     if not note:
         raise HTTPException(
@@ -150,10 +162,14 @@ async def update_review_note(
 async def delete_review_note(
     job_id: str,
     note_id: str,
-    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.ADMIN)),
+    current_user: User = Depends(require_roles(UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.PROJECT_MANAGER, UserRole.ADMIN)),
     db: AsyncIOMotorDatabase = Depends(get_database),
 ):
     """Delete a review note. Only the note owner can delete."""
+    job = await db.jobs.find_one({"_id": job_id}, {"_id": 1, "organization_id": 1, "project_id": 1, "client_id": 1})
+    if not job:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Job not found")
+    await assert_job_in_user_org(job, current_user, db)
     note = await db.review_notes.find_one({"_id": note_id, "job_id": job_id})
     if not note:
         raise HTTPException(
diff --git a/backend/app/api/v1/routes_vtt_versions.py b/backend/app/api/v1/routes_vtt_versions.py
index 90ce300..d213949 100644
--- a/backend/app/api/v1/routes_vtt_versions.py
+++ b/backend/app/api/v1/routes_vtt_versions.py
@@ -4,7 +4,7 @@ from fastapi import APIRouter, Depends, HTTPException, Query, Request, status
 from motor.motor_asyncio import AsyncIOMotorDatabase
 
 from ...core.database import get_database
-from ...core.dependencies import require_roles
+from ...core.dependencies import assert_job_in_user_org, require_roles
 from ...models.user import User, UserRole
 from ...models.vtt_version import VttDiffResponse, VttKind, VttVersionListResponse, VttVersionSummary
 from ...services import vtt_versioning
@@ -15,7 +15,16 @@ from ...core.config import settings
 
 router = APIRouter(prefix="/jobs", tags=["vtt-versions"])
 
-_EDITABLE_ROLES = (UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.ADMIN)
+_EDITABLE_ROLES = (UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION, UserRole.PROJECT_MANAGER, UserRole.ADMIN)
+
+
+async def _assert_job_access(job_id: str, user: User, db) -> None:
+    job = await db.jobs.find_one(
+        {"_id": job_id}, {"_id": 1, "organization_id": 1, "project_id": 1, "client_id": 1}
+    )
+    if not job:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Job not found")
+    await assert_job_in_user_org(job, user, db)
 
 
 @router.get("/{job_id}/vtt/versions", response_model=VttVersionListResponse)
@@ -29,6 +38,7 @@ async def list_vtt_versions(
     db: AsyncIOMotorDatabase = Depends(get_database),
 ):
     """List all VTT versions for a job/lang/kind, newest first."""
+    await _assert_job_access(job_id, current_user, db)
     return await vtt_versioning.list_versions(db, job_id, lang, kind, skip, limit)
 
 
@@ -42,6 +52,7 @@ async def get_vtt_version(
     db: AsyncIOMotorDatabase = Depends(get_database),
 ):
     """Get full VTT content for a specific version."""
+    await _assert_job_access(job_id, current_user, db)
     v = await vtt_versioning.get_version(db, job_id, lang, kind, version)
     if not v:
         raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Version not found")
@@ -72,6 +83,7 @@ async def diff_vtt_versions(
     db: AsyncIOMotorDatabase = Depends(get_database),
 ):
     """Line-level diff between two versions of a VTT file."""
+    await _assert_job_access(job_id, current_user, db)
     v_from = await vtt_versioning.get_version(db, job_id, lang, kind, from_version)
     v_to = await vtt_versioning.get_version(db, job_id, lang, kind, to_version)
     if not v_from:
@@ -100,6 +112,7 @@ async def restore_vtt_version(
     Non-destructive: creates a new version entry whose content mirrors the old one,
     then overwrites the live GCS file.
     """
+    await _assert_job_access(job_id, current_user, db)
     src = await vtt_versioning.get_version(db, job_id, lang, kind, version)
     if not src:
         raise HTTPException(status_code=404, detail="Version not found")
diff --git a/backend/app/api/v1/routes_websockets.py b/backend/app/api/v1/routes_websockets.py
index b1e3180..623e669 100644
--- a/backend/app/api/v1/routes_websockets.py
+++ b/backend/app/api/v1/routes_websockets.py
@@ -18,8 +18,9 @@ from ...services.websocket import (
     ConnectionManager
 )
 from ...models.job import Job
+from ...models.user import UserRole
 from ...core.database import get_database
-from ...core.dependencies import get_current_user
+from ...core.dependencies import get_current_user, get_user_org_ids
 
 logger = logging.getLogger(__name__)
 
@@ -68,23 +69,25 @@ async def websocket_job_status(
             await websocket.close(code=4004, reason="Job not found")
             return
         
-        # Check permissions - users can only access their own jobs unless they're admin/reviewer
-        user = await db["users"].find_one({"_id": user_id})
-        if not user:
-            try:
-                from bson import ObjectId
-                user = await db["users"].find_one({"_id": ObjectId(user_id)})
-            except Exception:
-                pass  # Invalid ObjectId format
-        
-        if not user:
+        # Verify user and check org membership
+        user_doc = await db["users"].find_one({"_id": user_id})
+        if not user_doc:
             await websocket.close(code=4001, reason="User not found")
             return
-            
-        # Check access permissions
-        if user["role"] == "client" and job.get("created_by") != user_id:
-            await websocket.close(code=4003, reason="Access denied")
-            return
+
+        from ...models.user import User
+        user_obj = User(**user_doc)
+
+        # ADMIN sees everything; others must belong to the job's org
+        if user_obj.role != UserRole.ADMIN:
+            org_ids = await get_user_org_ids(user_obj, db)
+            job_org = job.get("organization_id") or job.get("client_id")
+            if job_org and org_ids is not None and job_org not in org_ids:
+                logger.warning(
+                    f"WS: user {user_id} tried to subscribe to job {job_id} from a different org"
+                )
+                await websocket.close(code=4003, reason="Access denied")
+                return
         
         # Connect to job status updates
         await manager.connect_job_status(websocket, user_id, job_id)
diff --git a/backend/app/core/dependencies.py b/backend/app/core/dependencies.py
index 8184d80..c854e05 100644
--- a/backend/app/core/dependencies.py
+++ b/backend/app/core/dependencies.py
@@ -1,11 +1,9 @@
-from typing import Optional
 
 from fastapi import Depends, HTTPException, Request, status
 from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
 from motor.motor_asyncio import AsyncIOMotorDatabase
 
 from ..models.user import User, UserRole
-from .config import settings
 from .database import get_database
 from .security import decode_token
 
@@ -15,6 +13,57 @@ security = HTTPBearer()
 STAFF_ROLES = {UserRole.ADMIN}
 
 
+async def get_user_org_ids(user: User, db: AsyncIOMotorDatabase) -> list[str] | None:
+    """Return organization IDs accessible to the user, or None meaning unrestricted (ADMIN only)."""
+    if user.role == UserRole.ADMIN:
+        return None  # ADMIN sees everything
+    user_id = str(user.id)
+    membership_cursor = db.memberships.find({"user_id": user_id}, {"organization_id": 1})
+    org_ids = [doc["organization_id"] async for doc in membership_cursor]
+    if org_ids:
+        return org_ids
+    # Legacy: team membership
+    teams = await db.teams.find({"member_user_ids": user_id}, {"client_id": 1}).to_list(None)
+    client_ids = list({t["client_id"] for t in teams if t.get("client_id")})
+    if client_ids:
+        return client_ids
+    # PM legacy via pm_client_ids
+    if user.role == UserRole.PROJECT_MANAGER and user.pm_client_ids:
+        return list(user.pm_client_ids)
+    # No membership at all — return empty list (see nothing)
+    return []
+
+
+async def assert_job_in_user_org(job_doc: dict, user: User, db: AsyncIOMotorDatabase) -> None:
+    """Raise 404 if the user cannot access the job's organization."""
+    if user.role == UserRole.ADMIN:
+        return
+    org_ids = await get_user_org_ids(user, db)
+    if org_ids is None:
+        return  # unrestricted (should only happen for ADMIN but defensive)
+
+    # Check explicit organization_id field first (new jobs)
+    job_org = job_doc.get("organization_id")
+    if job_org:
+        if job_org not in org_ids:
+            raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Job not found")
+        return
+
+    # Fallback: derive org from project
+    project_id = job_doc.get("project_id")
+    if project_id:
+        project = await db.projects.find_one({"_id": project_id}, {"client_id": 1})
+        if project and project.get("client_id") in org_ids:
+            return
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Job not found")
+
+    # Legacy jobs with no project: only visible to the user who created them
+    if job_doc.get("client_id") == str(user.id):
+        return
+    # Admin-only for fully legacy jobs with no org context
+    raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Job not found")
+
+
 async def get_current_user(
     credentials: HTTPAuthorizationCredentials = Depends(security),
     db: AsyncIOMotorDatabase = Depends(get_database),
@@ -73,7 +122,7 @@ def require_roles(*required_roles: UserRole):
 async def get_current_user_optional(
     request: Request,
     db: AsyncIOMotorDatabase = Depends(get_database),
-) -> Optional[User]:
+) -> User | None:
     authorization: str = request.headers.get("Authorization")
     if not authorization:
         return None
@@ -104,7 +153,7 @@ async def get_current_user_optional(
 async def get_accessible_project_ids(
     user: User,
     db: AsyncIOMotorDatabase,
-) -> Optional[list[str]]:
+) -> list[str] | None:
     """
     Returns project IDs the user may access, or None meaning "see everything".
 
@@ -156,11 +205,8 @@ async def get_accessible_project_ids(
         ).to_list(None)
         return [str(p["_id"]) for p in projects]
 
-    # Staff with no team assignments → unrestricted until teams are configured
-    if user.role in {UserRole.REVIEWER, UserRole.LINGUIST, UserRole.PRODUCTION}:
-        return None
-
-    # CLIENT with no memberships and no teams → show nothing
+    # Staff with no team assignments → show nothing (no org context yet)
+    # Returning None here would give unrestricted cross-tenant access.
     return []
 
 
diff --git a/backend/app/models/audit_log.py b/backend/app/models/audit_log.py
index 7f4fa08..17c92d1 100644
--- a/backend/app/models/audit_log.py
+++ b/backend/app/models/audit_log.py
@@ -93,11 +93,14 @@ class AuditLog(BaseModel):
     
     id: Optional[PyObjectId] = Field(default_factory=lambda: str(ObjectId()), alias="_id")
     
+    # Tenant isolation
+    organization_id: Optional[str] = None
+
     # Core audit fields
     timestamp: datetime = Field(default_factory=datetime.utcnow)
     action: AuditAction
     severity: AuditLogSeverity = AuditLogSeverity.INFO
-    
+
     # Actor information
     user_id: Optional[PyObjectId] = None
     user_email: Optional[str] = None
diff --git a/backend/app/models/job.py b/backend/app/models/job.py
index 6a1effc..d308ed9 100644
--- a/backend/app/models/job.py
+++ b/backend/app/models/job.py
@@ -235,6 +235,7 @@ class Job(BaseModel):
     ai: Optional[AISection] = None
     error: Optional[dict[str, Any]] = None
     tts_rewrites: Optional[list[dict[str, Any]]] = None  # Track auto-rewritten TTS cues
+    organization_id: Optional[str] = None  # Denormalized client_id (org) for tenant isolation
     project_id: Optional[str] = None  # Platform project this job belongs to (Client → Project → Job)
     brand_context: Optional[str] = None  # Brand names present in the video for accurate product identification
     cost_tracker_project_id: Optional[str] = None  # External project ID for AI cost attribution
diff --git a/backend/app/models/review_note.py b/backend/app/models/review_note.py
index 4611264..0da37e4 100644
--- a/backend/app/models/review_note.py
+++ b/backend/app/models/review_note.py
@@ -10,6 +10,7 @@ class ReviewNote(BaseModel):
     """A timestamped note attached to a video asset during review."""
 
     id: Optional[str] = Field(None, alias="_id")
+    organization_id: Optional[str] = None  # Denormalized for tenant isolation
     job_id: str
     asset_key: str  # e.g., "en", "es", "en_accessible"
     timestamp_seconds: float  # Video timestamp when note was created
diff --git a/backend/app/models/vtt_version.py b/backend/app/models/vtt_version.py
index acecdfa..8a43a2d 100644
--- a/backend/app/models/vtt_version.py
+++ b/backend/app/models/vtt_version.py
@@ -14,6 +14,7 @@ class VttVersionActor(BaseModel):
 
 class VttVersion(BaseModel):
     id: Optional[str] = Field(None, alias="_id")
+    organization_id: Optional[str] = None  # Denormalized for tenant isolation
     job_id: str
     lang: str
     kind: VttKind
diff --git a/backend/app/services/audit_logger.py b/backend/app/services/audit_logger.py
index 39261b4..429fa10 100644
--- a/backend/app/services/audit_logger.py
+++ b/backend/app/services/audit_logger.py
@@ -47,7 +47,8 @@ class AuditLogger:
         details: Optional[Dict[str, Any]] = None,
         severity: AuditLogSeverity = AuditLogSeverity.INFO,
         success: bool = True,
-        error_message: Optional[str] = None
+        error_message: Optional[str] = None,
+        organization_id: Optional[str] = None,
     ) -> str:
         """
         Log an audit event.
@@ -77,6 +78,7 @@ class AuditLogger:
             action=action,
             severity=severity,
             description=description,
+            organization_id=organization_id,
             user_id=user.id if user else None,
             user_email=user.email if user else None,
             user_role=(user.role.value if hasattr(user.role, "value") else user.role) if user else None,
@@ -277,16 +279,23 @@ async def log_auth_failure(email: str, request: Request, reason: str):
     )
 
 
-async def log_job_action(action: AuditAction, job_id: str, user: User, request: Request, details: Optional[Dict] = None):
+async def log_job_action(
+    action: AuditAction,
+    job_id: str,
+    user: User,
+    request: Request,
+    details: Optional[Dict] = None,
+    organization_id: Optional[str] = None,
+):
     """Log job-related actions."""
     action_descriptions = {
         AuditAction.JOB_CREATE: "Job created",
-        AuditAction.JOB_APPROVE: "Job approved", 
+        AuditAction.JOB_APPROVE: "Job approved",
         AuditAction.JOB_REJECT: "Job rejected",
         AuditAction.JOB_CANCEL: "Job cancelled",
         AuditAction.JOB_UPDATE: "Job updated"
     }
-    
+
     await audit_logger.log_action(
         action=action,
         description=f"{action_descriptions.get(action, str(action))} by {user.email}",
@@ -294,7 +303,8 @@ async def log_job_action(action: AuditAction, job_id: str, user: User, request:
         request=request,
         resource_type="job",
         resource_id=job_id,
-        details=details
+        details=details,
+        organization_id=organization_id,
     )
 
 
diff --git a/backend/migrations/2026_05_add_organization_id.py b/backend/migrations/2026_05_add_organization_id.py
new file mode 100644
index 0000000..4f3dc0c
--- /dev/null
+++ b/backend/migrations/2026_05_add_organization_id.py
@@ -0,0 +1,125 @@
+"""
+Migration: add organization_id to jobs, review_notes, vtt_versions, audit_logs.
+
+Run once after deploying the multi-tenant isolation PR.
+
+Usage (inside the backend container):
+    python -m migrations.2026_05_add_organization_id
+
+Strategy:
+- Jobs with a project_id  → organization_id = project.client_id
+- Jobs with no project_id → organization_id stays None (legacy single-tenant data;
+                             the assert_job_in_user_org fallback handles these via
+                             client_id == user.id check)
+- review_notes / vtt_versions / audit_logs → copy organization_id from their parent job
+"""
+
+import asyncio
+import logging
+from motor.motor_asyncio import AsyncIOMotorClient
+
+from app.core.config import get_settings
+
+logger = logging.getLogger(__name__)
+logging.basicConfig(level=logging.INFO)
+
+
+async def run():
+    settings = get_settings()
+    client = AsyncIOMotorClient(settings.mongodb_url)
+    db = client[settings.mongodb_db_name]
+
+    # ── Step 1: backfill jobs ────────────────────────────────────────────────
+    logger.info("Backfilling organization_id on jobs…")
+
+    # Build project_id → client_id lookup
+    project_map: dict[str, str] = {}
+    async for proj in db.projects.find({}, {"_id": 1, "client_id": 1}):
+        project_map[str(proj["_id"])] = str(proj["client_id"])
+
+    jobs_updated = 0
+    async for job in db.jobs.find(
+        {"organization_id": {"$exists": False}, "project_id": {"$ne": None}},
+        {"_id": 1, "project_id": 1},
+    ):
+        pid = job.get("project_id")
+        if pid and pid in project_map:
+            await db.jobs.update_one(
+                {"_id": job["_id"]},
+                {"$set": {"organization_id": project_map[pid]}},
+            )
+            jobs_updated += 1
+
+    logger.info(f"Jobs updated: {jobs_updated}")
+
+    # ── Step 2: build job_id → organization_id for downstream tables ─────────
+    job_org_map: dict[str, str] = {}
+    async for job in db.jobs.find(
+        {"organization_id": {"$exists": True}},
+        {"_id": 1, "organization_id": 1},
+    ):
+        if job.get("organization_id"):
+            job_org_map[str(job["_id"])] = job["organization_id"]
+
+    # ── Step 3: backfill review_notes ────────────────────────────────────────
+    logger.info("Backfilling organization_id on review_notes…")
+    notes_updated = 0
+    async for note in db.review_notes.find(
+        {"organization_id": {"$exists": False}},
+        {"_id": 1, "job_id": 1},
+    ):
+        org = job_org_map.get(note.get("job_id", ""))
+        if org:
+            await db.review_notes.update_one(
+                {"_id": note["_id"]},
+                {"$set": {"organization_id": org}},
+            )
+            notes_updated += 1
+    logger.info(f"Review notes updated: {notes_updated}")
+
+    # ── Step 4: backfill vtt_versions ────────────────────────────────────────
+    logger.info("Backfilling organization_id on vtt_versions…")
+    vtt_updated = 0
+    async for v in db.vtt_versions.find(
+        {"organization_id": {"$exists": False}},
+        {"_id": 1, "job_id": 1},
+    ):
+        org = job_org_map.get(v.get("job_id", ""))
+        if org:
+            await db.vtt_versions.update_one(
+                {"_id": v["_id"]},
+                {"$set": {"organization_id": org}},
+            )
+            vtt_updated += 1
+    logger.info(f"VTT versions updated: {vtt_updated}")
+
+    # ── Step 5: backfill audit_logs ──────────────────────────────────────────
+    logger.info("Backfilling organization_id on audit_logs…")
+    audit_updated = 0
+    async for entry in db.audit_logs.find(
+        {"organization_id": {"$exists": False}, "resource_type": "job"},
+        {"_id": 1, "resource_id": 1},
+    ):
+        org = job_org_map.get(entry.get("resource_id", ""))
+        if org:
+            await db.audit_logs.update_one(
+                {"_id": entry["_id"]},
+                {"$set": {"organization_id": org}},
+            )
+            audit_updated += 1
+    logger.info(f"Audit logs updated: {audit_updated}")
+
+    # ── Step 6: create indexes ───────────────────────────────────────────────
+    logger.info("Creating organization_id indexes…")
+    await db.jobs.create_index([("organization_id", 1), ("created_at", -1)])
+    await db.review_notes.create_index([("organization_id", 1), ("job_id", 1)])
+    await db.vtt_versions.create_index([("organization_id", 1), ("job_id", 1)])
+    await db.audit_logs.create_index([("organization_id", 1), ("timestamp", -1)])
+    logger.info("Indexes created.")
+
+    client.close()
+    logger.info("Migration complete.")
+
+
+if __name__ == "__main__":
+    asyncio.run(run())
diff --git a/backend/tests/unit/test_cross_tenant_isolation.py b/backend/tests/unit/test_cross_tenant_isolation.py
new file mode 100644
index 0000000..b9bf060
--- /dev/null
+++ b/backend/tests/unit/test_cross_tenant_isolation.py
@@ -0,0 +1,191 @@
+"""
+Cross-tenant isolation tests.
+
+Verifies that assert_job_in_user_org raises 404 (not 403, to avoid information
+disclosure) when a user from org A tries to access a job belonging to org B.
+Also verifies ADMIN always passes and legacy (no organization_id) jobs fall back
+to project-based checks.
+"""
+
+import pytest
+from unittest.mock import AsyncMock, MagicMock, patch
+from fastapi import HTTPException
+
+from app.core.dependencies import assert_job_in_user_org, get_user_org_ids
+from app.models.user import User, UserRole
+
+
+# ── Helpers ────────────────────────────────────────────────────────────────────
+
+def _make_user(role: UserRole, user_id: str = "user-a") -> User:
+    return User(
+        **{
+            "_id": user_id,
+            "email": f"{user_id}@example.com",
+            "full_name": "Test User",
+            "role": role,
+            "is_active": True,
+        }
+    )
+
+
+def _make_db(memberships: list[dict], teams: list[dict] | None = None, project: dict | None = None):
+    """Return a mock AsyncIOMotorDatabase with memberships/teams/projects collections."""
+    db = MagicMock()
+
+    # memberships.find(...)
+    async def _mem_find(*args, **kwargs):
+        return memberships
+
+    mem_cursor = MagicMock()
+    mem_cursor.__aiter__ = AsyncMock(
+        return_value=iter(memberships)
+    )
+    # Use async list comprehension-compatible mock
+    async def _mem_iter():
+        for m in memberships:
+            yield m
+
+    db.memberships.find.return_value = _AsyncIterableMock(memberships)
+
+    # teams.find(...)
+    team_list = teams or []
+    db.teams.find.return_value = MagicMock()
+    db.teams.find.return_value.to_list = AsyncMock(return_value=team_list)
+
+    # projects.find_one(...)
+    db.projects.find_one = AsyncMock(return_value=project)
+
+    return db
+
+
+class _AsyncIterableMock:
+    """Supports `async for doc in cursor` pattern used in get_user_org_ids."""
+
+    def __init__(self, items: list):
+        self._items = items
+
+    def __aiter__(self):
+        return self._aiter()
+
+    async def _aiter(self):
+        for item in self._items:
+            yield item
+
+
+# ── get_user_org_ids ───────────────────────────────────────────────────────────
+
+class TestGetUserOrgIds:
+    @pytest.mark.asyncio
+    async def test_admin_returns_none(self):
+        user = _make_user(UserRole.ADMIN)
+        db = _make_db([])
+        result = await get_user_org_ids(user, db)
+        assert result is None
+
+    @pytest.mark.asyncio
+    async def test_staff_with_membership_returns_orgs(self):
+        user = _make_user(UserRole.LINGUIST)
+        db = _make_db([{"organization_id": "org-a"}, {"organization_id": "org-b"}])
+        result = await get_user_org_ids(user, db)
+        assert result == ["org-a", "org-b"]
+
+    @pytest.mark.asyncio
+    async def test_staff_no_membership_no_team_returns_empty(self):
+        user = _make_user(UserRole.REVIEWER)
+        db = _make_db([])
+        result = await get_user_org_ids(user, db)
+        assert result == []
+
+    @pytest.mark.asyncio
+    async def test_pm_legacy_pm_client_ids(self):
+        user = _make_user(UserRole.PROJECT_MANAGER)
+        user.pm_client_ids = ["client-x"]
+        db = _make_db([])
+        result = await get_user_org_ids(user, db)
+        assert result == ["client-x"]
+
+    @pytest.mark.asyncio
+    async def test_staff_falls_back_to_team_membership(self):
+        user = _make_user(UserRole.PRODUCTION)
+        db = _make_db(
+            memberships=[],  # no memberships
+            teams=[{"client_id": "client-y"}],
+        )
+        result = await get_user_org_ids(user, db)
+        assert result == ["client-y"]
+
+
+# ── assert_job_in_user_org ─────────────────────────────────────────────────────
+
+class TestAssertJobInUserOrg:
+    @pytest.mark.asyncio
+    async def test_admin_always_passes(self):
+        user = _make_user(UserRole.ADMIN)
+        job = {"_id": "job-1", "organization_id": "org-b"}
+        db = _make_db([])
+        await assert_job_in_user_org(job, user, db)  # must not raise
+
+    @pytest.mark.asyncio
+    async def test_same_org_passes(self):
+        user = _make_user(UserRole.LINGUIST)
+        db = _make_db([{"organization_id": "org-a"}])
+        job = {"_id": "job-1", "organization_id": "org-a"}
+        await assert_job_in_user_org(job, user, db)  # must not raise
+
+    @pytest.mark.asyncio
+    async def test_cross_org_raises_404(self):
+        user = _make_user(UserRole.LINGUIST)
+        db = _make_db([{"organization_id": "org-a"}])
+        job = {"_id": "job-1", "organization_id": "org-b"}
+        with pytest.raises(HTTPException) as exc:
+            await assert_job_in_user_org(job, user, db)
+        assert exc.value.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_no_organization_id_project_fallback_passes(self):
+        user = _make_user(UserRole.REVIEWER)
+        db = _make_db(
+            memberships=[{"organization_id": "org-a"}],
+            project={"_id": "proj-1", "client_id": "org-a"},
+        )
+        job = {"_id": "job-1", "project_id": "proj-1"}  # no organization_id
+        await assert_job_in_user_org(job, user, db)
+
+    @pytest.mark.asyncio
+    async def test_no_organization_id_project_from_other_org_raises_404(self):
+        user = _make_user(UserRole.REVIEWER)
+        db = _make_db(
+            memberships=[{"organization_id": "org-a"}],
+            project={"_id": "proj-1", "client_id": "org-b"},
+        )
+        job = {"_id": "job-1", "project_id": "proj-1"}
+        with pytest.raises(HTTPException) as exc:
+            await assert_job_in_user_org(job, user, db)
+        assert exc.value.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_legacy_job_own_user_passes(self):
+        user = _make_user(UserRole.LINGUIST, user_id="user-a")
+        db = _make_db([])  # no memberships
+        job = {"_id": "job-1", "client_id": "user-a"}  # legacy: client_id = creator user_id
+        await assert_job_in_user_org(job, user, db)
+
+    @pytest.mark.asyncio
+    async def test_legacy_job_other_user_raises_404(self):
+        user = _make_user(UserRole.LINGUIST, user_id="user-a")
+        db = _make_db([])  # no memberships
+        job = {"_id": "job-1", "client_id": "user-b"}  # belongs to a different user
+        with pytest.raises(HTTPException) as exc:
+            await assert_job_in_user_org(job, user, db)
+        assert exc.value.status_code == 404
+
+    @pytest.mark.asyncio
+    async def test_user_with_no_org_context_denied_access_to_org_job(self):
+        """Staff with no memberships and no team cannot access any org-tagged job."""
+        user = _make_user(UserRole.PRODUCTION)
+        db = _make_db([])
+        job = {"_id": "job-1", "organization_id": "org-c"}
+        with pytest.raises(HTTPException) as exc:
+            await assert_job_in_user_org(job, user, db)
+        assert exc.value.status_code == 404