diff --git a/app/controllers/client.py b/app/controllers/client.py
index e9ab58337559fa03dad0eaba83d7ecc11a33f39d..e7a081b2e962b6190428f404c7ee4664b223bde6 100644
--- a/app/controllers/client.py
+++ b/app/controllers/client.py
@@ -1,6 +1,8 @@
 """Controllers for managing clients."""
 from werkzeug.exceptions import BadRequest
+
 from app.controllers.base import RestController
+from app.models.project import Project
 from app.models.client import Client
 from app.forms.client import ClientForm
 
@@ -9,6 +11,15 @@ class ClientController(RestController):
     """The client controller."""
     Model = Client
     constant_fields = ['sales_force_id']
+    filters = {
+        'project_id': lambda d: Project.id == d['project_id']}
+
+    def query(self, filter_data):
+        """Construct a query with additional joins based on filter_data."""
+        q = super(ClientController, self).query(filter_data)
+        if 'project_id' in filter_data:
+            q = q.join(Project, Project.client_id == Client.id)
+        return q
 
     def get_form(self, filter_data):
         """Return the client form."""
diff --git a/app/tests/test_client.py b/app/tests/test_client.py
index 0dca40928934543b353fbe29bf0f06f96325fca0..8cd245154b56f1450a59887c411da07eb3cf52ca 100644
--- a/app/tests/test_client.py
+++ b/app/tests/test_client.py
@@ -15,6 +15,20 @@ class TestClient(RestTestCase):
         model = self.env.client
         self._test_index()
 
+    def test_index_project_id(self):
+        """Tests /client/?project_id=... GET."""
+        # Construct a model that should be returned.
+        project = self.env.project
+        model = db.session.query(Client)\
+            .filter(Client.id == project.client_id)\
+            .first()
+        # Construct a model that should not be returned.
+        _ = self.env.client
+
+        response_data = self._test_index({'project_id': project.id})
+        self.assertEqual(len(response_data), 1)
+        self.assertEqual(response_data[0]['id'], model.id)
+
     def test_get(self):
         """Tests /client/<id> GET."""
         model = self.env.client