diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 4fec647..de3e560 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -645,6 +645,9 @@
 
    .. 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
index f40685c..ab27c48 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -216,6 +216,9 @@
 :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
index 82c441c..50535da 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -956,14 +956,9 @@
     # 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
new file mode 100644
index 0000000..a185ab5
--- /dev/null
+++ 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
index aa4756a..661d91a 100644
--- a/Modules/clinic/posixmodule.c.h
+++ b/Modules/clinic/posixmodule.c.h
@@ -6125,7 +6125,43 @@
 
 #endif /* defined(HAVE_PUTENV) && !defined(MS_WINDOWS) */
 
-#if defined(HAVE_UNSETENV)
+#if defined(MS_WINDOWS)
+
+PyDoc_STRVAR(os_unsetenv__doc__,
+"unsetenv($module, name, /)\n"
+"--\n"
+"\n"
+"Delete an environment variable.");
+
+#define OS_UNSETENV_METHODDEF    \
+    {"unsetenv", (PyCFunction)os_unsetenv, METH_O, os_unsetenv__doc__},
+
+static PyObject *
+os_unsetenv_impl(PyObject *module, PyObject *name);
+
+static PyObject *
+os_unsetenv(PyObject *module, PyObject *arg)
+{
+    PyObject *return_value = NULL;
+    PyObject *name;
+
+    if (!PyUnicode_Check(arg)) {
+        _PyArg_BadArgument("unsetenv", "argument", "str", arg);
+        goto exit;
+    }
+    if (PyUnicode_READY(arg) == -1) {
+        goto exit;
+    }
+    name = arg;
+    return_value = os_unsetenv_impl(module, name);
+
+exit:
+    return return_value;
+}
+
+#endif /* defined(MS_WINDOWS) */
+
+#if (defined(HAVE_UNSETENV) && !defined(MS_WINDOWS))
 
 PyDoc_STRVAR(os_unsetenv__doc__,
 "unsetenv($module, name, /)\n"
@@ -6157,7 +6193,7 @@
     return return_value;
 }
 
-#endif /* defined(HAVE_UNSETENV) */
+#endif /* (defined(HAVE_UNSETENV) && !defined(MS_WINDOWS)) */
 
 PyDoc_STRVAR(os_strerror__doc__,
 "strerror($module, code, /)\n"
@@ -8773,4 +8809,4 @@
 #ifndef OS__REMOVE_DLL_DIRECTORY_METHODDEF
     #define OS__REMOVE_DLL_DIRECTORY_METHODDEF
 #endif /* !defined(OS__REMOVE_DLL_DIRECTORY_METHODDEF) */
-/*[clinic end generated code: output=51ba5b9536420cea input=a9049054013a1b77]*/
+/*[clinic end generated code: output=6e739a2715712e88 input=a9049054013a1b77]*/
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 322c215..d2047d9 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -10163,7 +10163,49 @@
 #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