diff --git a/Doc/library/os.rst b/Doc/library/os.rst
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1495,15 +1495,18 @@
 .. function:: listdir(path='.')
 
    Return a list containing the names of the entries in the directory given by
-   *path* (default: ``'.'``).  The list is in arbitrary order.  It does not
-   include the special entries ``'.'`` and ``'..'`` even if they are present in
-   the directory.
-
-   This function can be called with a bytes or string argument, and returns
-   filenames of the same datatype.
+   *path*.  The list is in arbitrary order, and does not include the special
+   entries ``'.'`` and ``'..'`` even if they are present in the directory.
+
+   *path* may be either of type ``str`` or of type ``bytes``.  If *path*
+   is of type ``bytes``, the filenames returned will also be of type ``bytes``;
+   in all other circumstances, they will be of type ``str``.
 
    This function can also support :ref:`specifying an open file descriptor
-   <path_fd>` (referring to a directory).
+   <path_fd>`; the file descriptor must refer to a directory.
+
+   .. note::
+      To encode ``str`` filenames to ``bytes``, use :func:`~os.fsencode`.
 
    Availability: Unix, Windows.
 
diff --git a/Lib/test/test_posix.py b/Lib/test/test_posix.py
--- a/Lib/test/test_posix.py
+++ b/Lib/test/test_posix.py
@@ -447,17 +447,28 @@
             posix.chdir(os.curdir)
             self.assertRaises(OSError, posix.chdir, support.TESTFN)
 
+    requires_listdir = unittest.skipUnless(hasattr(posix, 'listdir'),
+                                           "test needs posix.listdir()")
+
+    @requires_listdir
     def test_listdir(self):
-        if hasattr(posix, 'listdir'):
-            self.assertTrue(support.TESTFN in posix.listdir(os.curdir))
+        self.assertTrue(support.TESTFN in posix.listdir(os.curdir))
 
+    @requires_listdir
     def test_listdir_default(self):
-        # When listdir is called without argument, it's the same as listdir(os.curdir)
-        if hasattr(posix, 'listdir'):
-            self.assertTrue(support.TESTFN in posix.listdir())
+        # When listdir is called without argument,
+        # it's the same as listdir(os.curdir).
+        self.assertTrue(support.TESTFN in posix.listdir())
 
-    @unittest.skipUnless(os.listdir in os.supports_fd, "test needs fd support for os.listdir()")
-    def test_flistdir(self):
+    @requires_listdir
+    def test_listdir_bytes(self):
+        # When listdir is called with a bytes object,
+        # the returned strings are of type bytes.
+        self.assertTrue(os.fsencode(support.TESTFN) in posix.listdir(b'.'))
+
+    @unittest.skipUnless(hasattr(posix, "listdir") and (posix.listdir in os.supports_fd),
+                         "test needs fd support for posix.listdir()")
+    def test_listdir_fd(self):
         f = posix.open(posix.getcwd(), posix.O_RDONLY)
         self.addCleanup(posix.close, f)
         self.assertEqual(
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -3272,14 +3272,16 @@
 
 
 PyDoc_STRVAR(posix_listdir__doc__,
-"listdir(path='.') -> list_of_strings\n\n\
-Return a list containing the names of the entries in the directory.\n\
-\n\
+"listdir(path='.') -> list_of_filenames\n\n\
+Return a list containing the names of the files in the directory.\n\
 The list is in arbitrary order.  It does not include the special\n\
 entries '.' and '..' even if they are present in the directory.\n\
 \n\
-path can always be specified as a string.\n\
-On some platforms, path may also be specified as an open file descriptor.\n\
+path can be specified as either str or bytes.  If path is bytes,\n\
+  the filenames returned will also be bytes; in all other circumstances\n\
+  the filenames returned will be str.\n\
+On some platforms, path may also be specified as an open file descriptor;\n\
+  the file descriptor must refer to a directory.\n\
   If this functionality is unavailable, using it raises NotImplementedError.");
 
 static PyObject *
@@ -3316,7 +3318,7 @@
     PyObject *v;
     DIR *dirp = NULL;
     struct dirent *ep;
-    int arg_is_unicode = 1;
+    int return_str; /* if false, return bytes */
 #endif
 
     memset(&path, 0, sizeof(path));
@@ -3538,11 +3540,6 @@
 #else
 
     errno = 0;
-    /* v is never read, so it does not need to be initialized yet. */
-    if (path.narrow && !PyArg_ParseTuple(args, "U:listdir", &v)) {
-        arg_is_unicode = 0;
-        PyErr_Clear();
-    }
 #ifdef HAVE_FDOPENDIR
     if (path.fd != -1) {
         /* closedir() closes the FD, so we duplicate it */
@@ -3555,6 +3552,8 @@
             goto exit;
         }
 
+        return_str = 1;
+
         Py_BEGIN_ALLOW_THREADS
         dirp = fdopendir(fd);
         Py_END_ALLOW_THREADS
@@ -3562,7 +3561,17 @@
     else
 #endif
     {
-        char *name = path.narrow ? path.narrow : ".";
+        char *name;
+        if (path.narrow) {
+            name = path.narrow;
+            /* only return bytes if they specified a bytes object */
+            return_str = !(PyBytes_Check(path.object));
+        }
+        else {
+            name = ".";
+            return_str = 1;
+        }
+
         Py_BEGIN_ALLOW_THREADS
         dirp = opendir(name);
         Py_END_ALLOW_THREADS
@@ -3593,7 +3602,7 @@
             (NAMLEN(ep) == 1 ||
              (ep->d_name[1] == '.' && NAMLEN(ep) == 2)))
             continue;
-        if (arg_is_unicode)
+        if (return_str)
             v = PyUnicode_DecodeFSDefaultAndSize(ep->d_name, NAMLEN(ep));
         else
             v = PyBytes_FromStringAndSize(ep->d_name, NAMLEN(ep));