diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index cf708407c290342..6720cd622d7c5b4 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -1001,8 +1001,8 @@ def test_stdlib_dir(self):
         if marker and not os.path.exists(marker):
             marker = None
         expected = os.path.dirname(marker) if marker else None
-        actual = sys._stdlib_dir
-        self.assertEqual(actual, expected)
+        self.assertEqual(os.path.normpath(sys._stdlib_dir),
+                         os.path.normpath(expected))
 
 
 @test.support.cpython_only
diff --git a/Misc/NEWS.d/next/Tests/2021-10-07-13-27-12.bpo-45403.7QiDvw.rst b/Misc/NEWS.d/next/Tests/2021-10-07-13-27-12.bpo-45403.7QiDvw.rst
new file mode 100644
index 000000000000000..e4d170996098017
--- /dev/null
+++ b/Misc/NEWS.d/next/Tests/2021-10-07-13-27-12.bpo-45403.7QiDvw.rst
@@ -0,0 +1,2 @@
+Fix test_sys.test_stdlib_dir() when Python is built outside the source tree:
+compare normalized paths. Patch by Victor Stinner.