diff --git a/Lib/os.py b/Lib/os.py
index a5e1d805556998..739292b60a783b 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -374,6 +374,14 @@ def walk(top, topdown=True, onerror=None, followlinks=False):
     """
     sys.audit("os.walk", top, topdown, onerror, followlinks)
 
+    try:
+        top_stat = stat(fspath(top))
+    except OSError:
+        pass  # non-existing: fall through, scandir will handle it via onerror
+    else:
+        if not path.isdir(fspath(top)) and not path.islink(fspath(top)):
+            raise NotADirectoryError(20, "Not a directory", top)
+
     stack = [fspath(top)]
     islink, join = path.islink, path.join
     while stack:
@@ -540,11 +548,14 @@ def _fwalk(stack, isbytes, topdown, onerror, follow_symlinks):
         stack.append((_fwalk_close, topfd))
         if not follow_symlinks:
             if isroot and not st.S_ISDIR(orig_st.st_mode):
-                return
+                raise NotADirectoryError(20, "Not a directory", toppath)
             if not path.samestat(orig_st, stat(topfd)):
                 return
 
-        scandir_it = scandir(topfd)
+        try:
+            scandir_it = scandir(topfd)
+        except NotADirectoryError:
+            raise NotADirectoryError(20, "Not a directory", toppath)
         dirs = []
         nondirs = []
         entries = None if topdown or follow_symlinks else []
diff --git a/Lib/test/test_os/test_os.py b/Lib/test/test_os/test_os.py
index 68cb32cd40be30..d8cdeef0c77b6f 100644
--- a/Lib/test/test_os/test_os.py
+++ b/Lib/test/test_os/test_os.py
@@ -1981,6 +1981,13 @@ def test_walk_above_recursion_limit(self):
         self.assertEqual(sorted(dirs), ["SUB1", "SUB2", "d"])
         self.assertEqual(all, expected)
 
+    def test_walk_on_file_raises_not_a_directory(self):
+        # gh-101420: os.walk() was silently returning [] when top is a
+        # regular file instead of raising NotADirectoryError.
+        with tempfile.NamedTemporaryFile() as f:
+            with self.assertRaises(NotADirectoryError):
+                list(os.walk(f.name))
+
 
 @unittest.skipUnless(hasattr(os, 'fwalk'), "Test needs os.fwalk()")
 class FwalkTests(WalkTests):
@@ -2038,6 +2045,19 @@ def test_yields_correct_dir_fd(self):
                 # check that listdir() returns consistent information
                 self.assertEqual(set(os.listdir(rootfd)), set(dirs) | set(files))
 
+    def test_fwalk_on_file_raises_not_a_directory(self):
+        with tempfile.NamedTemporaryFile() as f:
+            with self.assertRaises(NotADirectoryError):
+                list(os.fwalk(f.name, follow_symlinks=False))
+
+            # follow_symlinks=True: raised but with fd int as filename, must now have path
+            with self.assertRaises(NotADirectoryError) as ctx:
+                list(os.fwalk(f.name, follow_symlinks=True))
+            self.assertEqual(
+                ctx.exception.filename, f.name,
+                "filename should be the path string, not a raw fd integer"
+            )
+
     @unittest.skipIf(
         support.is_android, "dup return value is unpredictable on Android"
     )