diff --git a/backend/app/models/project.py b/backend/app/models/project.py
index 807ab867..6d841298 100644
--- a/backend/app/models/project.py
+++ b/backend/app/models/project.py
@@ -21,10 +21,10 @@ class ProjectManager:
     """Gestiona projectes: metadades a BD, fitxers a StorageService."""
 
     @classmethod
-    def create_project(cls, name: str = "Unnamed Project", storage=None) -> Dict[str, Any]:
+    def create_project(cls, name: str = "Unnamed Project", storage=None, user_id: str = None) -> Dict[str, Any]:
         project_id = str(uuid.uuid4())
         with get_session() as db:
-            proj = ProjectModel(id=project_id, name=name, status="created")
+            proj = ProjectModel(id=project_id, name=name, status="created", user_id=user_id)
             db.add(proj)
             db.commit()
             db.refresh(proj)
@@ -59,10 +59,12 @@ class ProjectManager:
             db.commit()
 
     @classmethod
-    def list_projects(cls, limit: int = 50) -> List[Dict[str, Any]]:
+    def list_projects(cls, limit: int = 50, user_id: str = None) -> List[Dict[str, Any]]:
         from sqlalchemy import select, desc
         with get_session() as db:
             stmt = select(ProjectModel).order_by(desc(ProjectModel.created_at)).limit(limit)
+            if user_id is not None:
+                stmt = stmt.where(ProjectModel.user_id == user_id)
             projects = db.execute(stmt).scalars().all()
             for p in projects:
                 db.expunge(p)
@@ -302,6 +304,7 @@ class ProjectManager:
         return {
             "id": proj.id,
             "project_id": proj.id,
+            "user_id": proj.user_id,
             "name": proj.name,
             "status": proj.status,
             "analysis_summary": proj.analysis_summary,
diff --git a/backend/tests/test_project_isolation.py b/backend/tests/test_project_isolation.py
new file mode 100644
index 00000000..605efb27
--- /dev/null
+++ b/backend/tests/test_project_isolation.py
@@ -0,0 +1,61 @@
+"""Tests d'aïllament de projectes per user_id."""
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _db(in_memory_db):
+    pass
+
+
+def _make_user(email, role='user'):
+    from backend.app.models.db_models import UserModel
+    from backend.app.db import get_session
+    with get_session() as db:
+        user = UserModel(email=email, name=email, role=role, status='active')
+        db.add(user)
+        db.commit()
+        db.refresh(user)
+        return user.id
+
+
+def test_list_projects_filtered_by_user():
+    from backend.app.models.project import ProjectManager
+    uid1 = _make_user('u1@test.com')
+    uid2 = _make_user('u2@test.com')
+
+    ProjectManager.create_project(name="U1-A", user_id=uid1)
+    ProjectManager.create_project(name="U1-B", user_id=uid1)
+    ProjectManager.create_project(name="U2-A", user_id=uid2)
+
+    u1_projects = ProjectManager.list_projects(user_id=uid1)
+    assert len(u1_projects) == 2
+    assert all(p['user_id'] == uid1 for p in u1_projects)
+
+    u2_projects = ProjectManager.list_projects(user_id=uid2)
+    assert len(u2_projects) == 1
+    assert u2_projects[0]['name'] == 'U2-A'
+
+
+def test_list_projects_no_filter_returns_all():
+    from backend.app.models.project import ProjectManager
+    uid1 = _make_user('all1@test.com')
+    uid2 = _make_user('all2@test.com')
+    ProjectManager.create_project(name="P1", user_id=uid1)
+    ProjectManager.create_project(name="P2", user_id=uid2)
+    all_projects = ProjectManager.list_projects(user_id=None)
+    assert len(all_projects) >= 2
+
+
+def test_create_project_assigns_user_id():
+    from backend.app.models.project import ProjectManager
+    uid = _make_user('owner@test.com')
+    proj = ProjectManager.create_project(name="Owned", user_id=uid)
+    assert proj['user_id'] == uid
+
+
+def test_to_dict_includes_user_id():
+    from backend.app.models.project import ProjectManager
+    uid = _make_user('dict@test.com')
+    proj = ProjectManager.create_project(name="DictTest", user_id=uid)
+    assert 'user_id' in proj
+    assert proj['user_id'] == uid