diff --git a/Doc/library/os.rst b/Doc/library/os.rst
@@ -645,9 +645,6 @@ process and user.
 
    .. availability:: most flavors of Unix, Windows.
 
-   .. versionchanged:: 3.9
-      The function is now also available on Windows.
-
 
 .. _os-newstreams:
 

diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
@@ -216,9 +216,6 @@ Exposed the Linux-specific :func:`os.pidfd_open` (:issue:`38692`) and
 :data:`os.P_PIDFD` (:issue:`38713`) for process management with file
 descriptors.
 
-The :func:`os.unsetenv` function is now also available on Windows.
-(Contributed by Victor Stinner in :issue:`39413`.)
-
 poplib
 ------
 

diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
@@ -956,9 +956,14 @@ def test_environb(self):
     # On OS X < 10.6, unsetenv() doesn't return a value (bpo-13415).
     @support.requires_mac_ver(10, 6)
     def test_unset_error(self):
-        # "=" is not allowed in a variable name
-        key = 'key='
-        self.assertRaises(OSError, os.environ.__delitem__, key)
+        if sys.platform == "win32":
+            # an environment variable is limited to 32,767 characters
+            key = 'x' * 50000
+            self.assertRaises(ValueError, os.environ.__delitem__, key)
+        else:
+            # "=" is not allowed in a variable name
+            key = 'key='
+            self.assertRaises(OSError, os.environ.__delitem__, key)
 
     def test_key_type(self):
         missing = 'missingkey'

diff --git a/Misc/NEWS.d/next/Library/2020-01-21-15-48-35.bpo-39413.7XYDM8.rst b/Misc/NEWS.d/next/Library/2020-01-21-15-48-35.bpo-39413.7XYDM8.rst
diff --git a/Modules/clinic/posixmodule.c.h b/Modules/clinic/posixmodule.c.h
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
@@ -10180,51 +10180,7 @@ os_putenv_impl(PyObject *module, PyObject *name, PyObject *value)
 #endif /* HAVE_PUTENV */
 
 
-#ifdef MS_WINDOWS
-/*[clinic input]
-os.unsetenv
-    name: unicode
-    /
-
-Delete an environment variable.
-[clinic start generated code]*/
-
-static PyObject *
-os_unsetenv_impl(PyObject *module, PyObject *name)
-/*[clinic end generated code: output=54c4137ab1834f02 input=4d6a1747cc526d2f]*/
-{
-    /* PyUnicode_AsWideCharString() rejects embedded null characters */
-    wchar_t *name_str = PyUnicode_AsWideCharString(name, NULL);
-    if (name_str == NULL) {
-        return NULL;
-    }
-
-    BOOL ok = SetEnvironmentVariableW(name_str, NULL);
-    PyMem_Free(name_str);
-
-    if (!ok) {
-        return PyErr_SetFromWindowsErr(0);
-    }
-
-#ifdef PY_PUTENV_DICT
-    /* Remove the key from putenv_dict;
-     * this will cause it to be collected.  This has to
-     * happen after the real unsetenv() call because the
-     * old value was still accessible until then.
-     */
-    if (PyDict_DelItem(_posixstate(module)->putenv_dict, name)) {
-        /* really not much we can do; just leak */
-        if (!PyErr_ExceptionMatches(PyExc_KeyError)) {
-            return NULL;
-        }
-        PyErr_Clear();
-    }
-#endif
-
-    Py_RETURN_NONE;
-}
-/* repeat !defined(MS_WINDOWS) to workaround an Argument Clinic issue */
-#elif defined(HAVE_UNSETENV) && !defined(MS_WINDOWS)
+#ifdef HAVE_UNSETENV
 /*[clinic input]
 os.unsetenv
     name: FSConverter