diff --git a/Doc/library/os.rst b/Doc/library/os.rst
@@ -645,6 +645,9 @@ 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
@@ -216,6 +216,9 @@ 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
@@ -956,14 +956,9 @@ 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):
-        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)
+        # "=" 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
@@ -0,0 +1 @@
+The :func:`os.unsetenv` function is now also available on Windows.
diff --git a/Modules/clinic/posixmodule.c.h b/Modules/clinic/posixmodule.c.h
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
@@ -10163,7 +10163,49 @@ os_putenv_impl(PyObject *module, PyObject *name, PyObject *value)
 #endif /* HAVE_PUTENV */
 
 
-#ifdef HAVE_UNSETENV
+#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);
+    }
+
+    /* Remove the key from posix_putenv_garbage;
+     * 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)->posix_putenv_garbage, name)) {
+        /* really not much we can do; just leak */
+        if (!PyErr_ExceptionMatches(PyExc_KeyError)) {
+            return NULL;
+        }
+        PyErr_Clear();
+    }
+
+    Py_RETURN_NONE;
+}
+/* repeat !defined(MS_WINDOWS) to workaround an Argument Clinic issue */
+#elif defined(HAVE_UNSETENV) && !defined(MS_WINDOWS)
 /*[clinic input]
 os.unsetenv
     name: FSConverter
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		The :func:`os.unsetenv` function is now also available on Windows.