diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index efa28ea952b..d06f08ed843 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -423,6 +423,15 @@ class EnvironTests(mapping_tests.BasicTestMappingProtocol):
         value_str = value.decode(sys.getfilesystemencoding(), 'surrogateescape')
         self.assertEqual(os.environ['bytes'], value_str)
 
+    def test_unset_error(self):
+        if sys.platform == "win32":
+            # an environment variable is limited to 32,767 characters
+            key = 'x' * 50000
+        else:
+            # "=" is not allowed in a variable name
+            key = 'key='
+        self.assertRaises(OSError, os.environ.__delitem__, key)
+
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 0b228ef14bc..63508182750 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -83,6 +83,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #13415: os.unsetenv() doesn't ignore errors anymore.
+
 - Issue #13322: Fix BufferedWriter.write() to ensure that BlockingIOError is
   raised when the wrapped raw file is non-blocking and the write would block.
   Previous code assumed that the raw write() would raise BlockingIOError, but
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index a27ac7111dc..728644534d4 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6106,6 +6106,12 @@ posix_putenv(PyObject *self, PyObject *args)
        PyBytes_FromStringAndSize does not count that */
 #ifdef MS_WINDOWS
     len = wcslen(s1) + wcslen(s2) + 2;
+    if (_MAX_ENV < (len - 1)) {
+        PyErr_Format(PyExc_ValueError,
+                     "the environment variable is longer than %u characters",
+                     _MAX_ENV);
+        goto error;
+    }
     newstr = PyUnicode_FromUnicode(NULL, (int)len - 1);
 #else
     len = PyBytes_GET_SIZE(os1) + PyBytes_GET_SIZE(os2) + 2;
@@ -6177,42 +6183,30 @@ Delete an environment variable.");
 static PyObject *
 posix_unsetenv(PyObject *self, PyObject *args)
 {
-#ifdef MS_WINDOWS
-    char *s1;
-
-    if (!PyArg_ParseTuple(args, "s:unsetenv", &s1))
-        return NULL;
-#else
     PyObject *os1;
     char *s1;
+    int err;
 
     if (!PyArg_ParseTuple(args, "O&:unsetenv",
                           PyUnicode_FSConverter, &os1))
         return NULL;
     s1 = PyBytes_AsString(os1);
-#endif
 
-    unsetenv(s1);
+    err = unsetenv(s1);
+    if (err)
+        return posix_error();
 
     /* 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(posix_putenv_garbage,
-#ifdef MS_WINDOWS
-                       PyTuple_GET_ITEM(args, 0)
-#else
-                       os1
-#endif
-                       )) {
+    if (PyDict_DelItem(posix_putenv_garbage, os1)) {
         /* really not much we can do; just leak */
         PyErr_Clear();
     }
 
-#ifndef MS_WINDOWS
     Py_DECREF(os1);
-#endif
     Py_RETURN_NONE;
 }
 #endif /* unsetenv */