diff --git a/tools/check_test_registration.py b/tools/check_test_registration.py
index 83311cb20..0fb47521a 100644
--- a/tools/check_test_registration.py
+++ b/tools/check_test_registration.py
@@ -4,7 +4,7 @@ import subprocess
 from pathlib import Path
 
 TEST_OUTPUT = r"tests[\\/]"
-COMMAND = re.compile(r"^\s*(add_test|set_tests_properties)\s*\(", re.M | re.I)
+COMMAND = re.compile(r"^\s*(add_test|set_tests_properties|subdirs)\s*\(", re.M | re.I)
 ARGUMENT = re.compile(r'"(?:\\.|[^"\\])*"|\[(?P<equals>=*)\[.*?\](?P=equals)\]|[^\s()]+', re.S)
 
 KNOWN_UNRUN = set()
@@ -63,10 +63,19 @@ def commands(content):
 
 def registered(build):
     names = set()
-    for file in Path(build).rglob("CTestTestfile.cmake"):
+    pending = [Path(build).resolve()]
+    visited = set()
+    while pending:
+        directory = pending.pop().resolve()
+        file = directory / "CTestTestfile.cmake"
+        if directory in visited or not file.is_file():
+            continue
+        visited.add(directory)
         tests, disabled = {}, {}
         for command, arguments in commands(file.read_text(encoding="utf-8", errors="replace")):
-            if command == "add_test" and arguments:
+            if command == "subdirs":
+                pending.extend(directory / argument for argument in arguments)
+            elif command == "add_test" and arguments:
                 tests[arguments[0]] = arguments[1:]
             elif command == "set_tests_properties" and "PROPERTIES" in arguments:
                 properties = arguments.index("PROPERTIES")
diff --git a/tools/tests/test_test_registration.py b/tools/tests/test_test_registration.py
index 5a4649281..3d91bf0d6 100644
--- a/tools/tests/test_test_registration.py
+++ b/tools/tests/test_test_registration.py
@@ -80,6 +80,45 @@ class RegistrationTests(unittest.TestCase):
         names = self.registered(r'add_test("bounds" "C:\\build\\tests\\buffer_bounds_tests.exe")' + '\n')
         self.assertIn("buffer_bounds_tests", names)
 
+    def test_unreferenced_registration_directory_does_not_count(self):
+        with tempfile.TemporaryDirectory() as directory:
+            build = Path(directory)
+            (build / "CTestTestfile.cmake").write_text("", encoding="utf-8")
+            stale = build / "removed"
+            stale.mkdir()
+            (stale / "CTestTestfile.cmake").write_text(
+                'add_test("bounds" "/build/tests/buffer_bounds_tests")\n', encoding="utf-8")
+            self.assertNotIn("buffer_bounds_tests", check_test_registration.registered(build))
+            (build / "CTestTestfile.cmake").unlink()
+            self.assertNotIn("buffer_bounds_tests", check_test_registration.registered(build))
+
+    def test_only_reachable_nested_directories_are_registered(self):
+        with tempfile.TemporaryDirectory() as directory:
+            build = Path(directory)
+            active = build / "active tests"
+            nested = active / "nested"
+            removed = active / "removed"
+            nested.mkdir(parents=True)
+            removed.mkdir()
+            (build / "CTestTestfile.cmake").write_text('subdirs("active tests")\n', encoding="utf-8")
+            (active / "CTestTestfile.cmake").write_text('subdirs([=[nested]=])\n', encoding="utf-8")
+            for path, name in ((nested, "active_tests"), (removed, "removed_tests")):
+                (path / "CTestTestfile.cmake").write_text(
+                    f'add_test("check" "/build/tests/{name}")\n', encoding="utf-8")
+            self.assertEqual(check_test_registration.registered(build), {"active_tests"})
+
+    def test_external_registration_directory_is_followed(self):
+        with tempfile.TemporaryDirectory() as directory:
+            root = Path(directory)
+            build, external = root / "build", root / "external"
+            build.mkdir()
+            external.mkdir()
+            (build / "CTestTestfile.cmake").write_text(
+                f'subdirs("{external.as_posix()}")\n', encoding="utf-8")
+            (external / "CTestTestfile.cmake").write_text(
+                'add_test("bounds" "/build/tests/buffer_bounds_tests")\n', encoding="utf-8")
+            self.assertEqual(check_test_registration.registered(build), {"buffer_bounds_tests"})
+
 
 if __name__ == "__main__":
     unittest.main()