diff --git a/tests/__init__.py b/tests/__init__.py
index e69de29bb..ff68cee43 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -0,0 +1,19 @@
+"""Pin the GTK / GDK introspection versions for the repo-root test suite.
+
+``python3 -m unittest discover -s tests`` imports this package before any test
+module under ``tests/``, so requiring the versions here pins the whole suite to
+the GTK 3 / GDK 3 stack a real Gramps GUI session uses. A test that imports a
+``gramps.gui.*`` module directly never runs the launcher's own
+``require_version``; without this the GI stack can resolve to GTK 4 on a host
+where that is the default — emitting ``PyGIWarning`` and risking the wrong stack.
+"""
+
+try:
+    import gi
+
+    gi.require_version("Gdk", "3.0")
+    gi.require_version("Gtk", "3.0")
+except Exception:
+    # No PyGObject, or the 3.0 typelibs are unavailable — leave the environment
+    # untouched; this only fixes the version when it can.
+    pass
diff --git a/tests/test_gtk_version_pin.py b/tests/test_gtk_version_pin.py
new file mode 100644
index 000000000..3d9c32633
--- /dev/null
+++ b/tests/test_gtk_version_pin.py
@@ -0,0 +1,18 @@
+"""Guard: the repo-root test suite pins GTK/GDK to 3 (via tests/__init__.py)."""
+
+import unittest
+
+
+class GtkGdkVersionPin(unittest.TestCase):
+    def test_suite_pins_gtk_and_gdk_to_3(self):
+        try:
+            import gi
+        except ImportError:
+            self.skipTest("PyGObject not available")
+        repo = gi.Repository.get_default()
+        if "3.0" not in repo.enumerate_versions("Gtk") or \
+           "3.0" not in repo.enumerate_versions("Gdk"):
+            self.skipTest("GTK/GDK 3.0 introspection typelibs not installed")
+        # Importing the tests package ran tests/__init__.py, which must pin both.
+        self.assertEqual(gi.get_required_version("Gtk"), "3.0")
+        self.assertEqual(gi.get_required_version("Gdk"), "3.0")