diff --git a/backend/app/crud/onboarding.py b/backend/app/crud/onboarding.py
index 7cfdddb6f..e754414d0 100644
--- a/backend/app/crud/onboarding.py
+++ b/backend/app/crud/onboarding.py
@@ -9,6 +9,7 @@
     get_project_by_name,
     get_user_by_email,
 )
+from app.crud.user_project import add_user_to_project
 from app.models import (
     APIKey,
     Credential,
@@ -90,6 +91,19 @@ def onboard_project(
         session.add(user)
         session.flush()
 
+    _, mapping_status = add_user_to_project(
+        session=session,
+        email=user.email,
+        organization_id=organization.id,
+        project_id=project.id,
+        full_name=onboard_in.user_name,
+    )
+    if mapping_status == "different_project":
+        raise HTTPException(
+            status_code=409,
+            detail=(f"User '{user.email}' is already associated with another project"),
+        )
+
     raw_key, key_prefix, key_hash = api_key_manager.generate()
 
     api_key = APIKey(
diff --git a/backend/app/tests/crud/test_onboarding.py b/backend/app/tests/crud/test_onboarding.py
index a082fa75e..b9514dbf0 100644
--- a/backend/app/tests/crud/test_onboarding.py
+++ b/backend/app/tests/crud/test_onboarding.py
@@ -9,6 +9,7 @@
     get_user_by_email,
     get_organization_by_id,
 )
+from app.crud.user_project import add_user_to_project
 from app.models import (
     OnboardingRequest,
     OnboardingResponse,
@@ -151,6 +152,33 @@ def test_onboard_project_existing_user(db: Session) -> None:
     assert project is not None
 
 
+def test_onboard_project_user_already_in_another_project(db: Session) -> None:
+    """Onboarding must 409 when the user is already mapped to a different project."""
+    existing_project = create_test_project(db)
+    existing_user = create_random_user(db)
+
+    add_user_to_project(
+        session=db,
+        email=existing_user.email,
+        organization_id=existing_project.organization_id,
+        project_id=existing_project.id,
+    )
+    db.commit()
+
+    onboard_request = OnboardingRequest(
+        organization_name="TestOrgOnboardDifferent",
+        project_name="TestProjectOnboardDifferent",
+        email=existing_user.email,
+        password=random_lower_string(),
+    )
+
+    with pytest.raises(HTTPException) as exc_info:
+        onboard_project(session=db, onboard_in=onboard_request)
+
+    assert exc_info.value.status_code == 409
+    assert "already associated with another project" in str(exc_info.value.detail)
+
+
 def test_onboard_project_duplicate_project_name(db: Session) -> None:
     """Test that onboarding fails when project name already exists in organization."""
     # Create existing project