diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index f8bc865ee82..52eb7d05ed1 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -252,7 +252,7 @@ class _AssertWarnsContext(_AssertRaisesBaseContext):
     def __enter__(self):
         # The __warningregistry__'s need to be in a pristine state for tests
         # to work properly.
-        for v in sys.modules.values():
+        for v in list(sys.modules.values()):
             if getattr(v, '__warningregistry__', None):
                 v.__warningregistry__ = {}
         self.warnings_manager = warnings.catch_warnings(record=True)
diff --git a/Lib/unittest/test/test_case.py b/Lib/unittest/test/test_case.py
index f855c4dc00b..3dedcbe6aad 100644
--- a/Lib/unittest/test/test_case.py
+++ b/Lib/unittest/test/test_case.py
@@ -8,6 +8,7 @@ import logging
 import warnings
 import weakref
 import inspect
+import types
 
 from copy import deepcopy
 from test import support
@@ -1350,6 +1351,20 @@ test case
             pass
         self.assertRaises(TypeError, self.assertWarnsRegex, MyWarn, lambda: True)
 
+    def testAssertWarnsModifySysModules(self):
+        # bpo-29620: handle modified sys.modules during iteration
+        class Foo(types.ModuleType):
+            @property
+            def __warningregistry__(self):
+                sys.modules['@bar@'] = 'bar'
+
+        sys.modules['@foo@'] = Foo('foo')
+        try:
+            self.assertWarns(UserWarning, warnings.warn, 'expected')
+        finally:
+            del sys.modules['@foo@']
+            del sys.modules['@bar@']
+
     def testAssertRaisesRegexMismatch(self):
         def Stub():
             raise Exception('Unexpected')
diff --git a/Misc/NEWS.d/next/Library/2018-08-21-16-20-33.bpo-29620.xxx666.rst b/Misc/NEWS.d/next/Library/2018-08-21-16-20-33.bpo-29620.xxx666.rst
new file mode 100644
index 00000000000..d781919504e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-08-21-16-20-33.bpo-29620.xxx666.rst
@@ -0,0 +1,3 @@
+:func:`~unittest.TestCase.assertWarns` no longer raises a ``RuntimeException``
+when accessing a module's ``__warningregistry__`` causes importation of a new
+module, or when a new module is imported in another thread. Patch by Kernc.