diff --git a/Lib/os.py b/Lib/os.py
index e293ecae7f..7699663884 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -25,6 +25,7 @@ and opendir), and leave all pathname manipulation to os.path
 import abc
 import sys, errno
 import stat as st
+import threading
 
 _names = sys.builtin_module_names
 
@@ -672,6 +673,7 @@ class _Environ(MutableMapping):
         self.putenv = putenv
         self.unsetenv = unsetenv
         self._data = data
+        self._lock = threading.Lock()
 
     def __getitem__(self, key):
         try:
@@ -697,7 +699,9 @@ class _Environ(MutableMapping):
             raise KeyError(key) from None
 
     def __iter__(self):
-        for key in self._data:
+        with self._lock:
+            keys = list(self._data)
+        for key in keys:
             yield self.decodekey(key)
 
     def __len__(self):
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 746b3f8be8..9cbe4b90d1 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -828,6 +828,21 @@ class EnvironTests(mapping_tests.BasicTestMappingProtocol):
         self.assertIs(cm.exception.args[0], missing)
         self.assertTrue(cm.exception.__suppress_context__)
 
+    def test_iter_error_when_os_environ_changes(self):
+        def _iter_environ_change():
+            for key, value in os.environ.items():
+                yield key, value
+
+        iter_environ = _iter_environ_change()
+        key, value = next(iter_environ)  # start iteration over os.environ
+
+        # add a new key in os.environ mapping
+        new_key = "__{}".format(key)
+        os.environ[new_key] = value
+
+        next(iter_environ)  # force iteration over modified mapping
+        self.assertEqual(os.environ[new_key], value)
+
 
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""