diff --git a/.test-infra/tools/stale_cleaner.py b/.test-infra/tools/stale_cleaner.py
index 3c8b21c806d8..6557949a0545 100644
--- a/.test-infra/tools/stale_cleaner.py
+++ b/.test-infra/tools/stale_cleaner.py
@@ -350,8 +350,10 @@ def _delete_resource(self, resource_name: str) -> None:
         self.client = pubsub_v1.SubscriberClient()
         print(f"{self.clock()} - Deleting PubSub subscription {resource_name}")
         with self.client:
-            subscription_path = self.client.subscription_path(self.project_id, resource_name)
-            self.client.delete_subscription(request={"subscription": subscription_path})
+            # resource_name from list_subscriptions() is already a fully-qualified
+            # path (e.g. "projects/<project>/subscriptions/<sub-id>"). Passing it
+            # to subscription_path() would double-prefix and cause a 400 error.
+            self.client.delete_subscription(request={"subscription": resource_name})
 
 def clean_pubsub_topics():
     """ Clean up stale PubSub topics in the specified GCP project.
@@ -439,4 +441,4 @@ def clean_pubsub_subscriptions():
     clean_pubsub_topics()
 
     # Clean up stale PubSub subscriptions
-    clean_pubsub_subscriptions()
+    clean_pubsub_subscriptions()
\ No newline at end of file
diff --git a/.test-infra/tools/test_stale_cleaner.py b/.test-infra/tools/test_stale_cleaner.py
index 08cdab39b85f..8a43d8705823 100644
--- a/.test-infra/tools/test_stale_cleaner.py
+++ b/.test-infra/tools/test_stale_cleaner.py
@@ -484,17 +484,22 @@ def test_active_resources_detached_subscriptions(self):
             self.assertEqual(len(active), 1)
 
     def test_delete_resource(self):
-        """Test _delete_resource method."""
-        sub_name = "test-sub-to-delete"
-        subscription_path = f"projects/{self.project_id}/subscriptions/{sub_name}"
-        self.mock_subscriber_client.subscription_path.return_value = subscription_path
+        """Test _delete_resource method.
+
+        list_subscriptions() returns subscription names as fully-qualified
+        paths (e.g. "projects/<project>/subscriptions/<sub-id>"), so
+        _delete_resource must pass them directly to delete_subscription
+        without re-prefixing via subscription_path(). Double-prefixing
+        causes InvalidArgument 400 and is the cause of issue #39772.
+        """
+        sub_path = f"projects/{self.project_id}/subscriptions/test-sub-to-delete"
 
         with SilencePrint():
-            self.cleaner._delete_resource(sub_name)
+            self.cleaner._delete_resource(sub_path)
 
-        self.mock_subscriber_client.subscription_path.assert_called_once_with(self.project_id, sub_name)
+        self.mock_subscriber_client.subscription_path.assert_not_called()
         self.mock_subscriber_client.delete_subscription.assert_called_once_with(
-            request={'subscription': subscription_path}
+            request={'subscription': sub_path}
         )