diff --git a/Lib/test/test_warnings.py b/Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py
+++ b/Lib/test/test_warnings.py
@@ -365,16 +365,34 @@ class WarnTests(BaseTest):
             """Warning with a bad format string for __str__."""
             def __str__(self):
                 return ("A bad formatted string %(err)" %
                         {"err" : "there is no %(err)s"})
 
         with self.assertRaises(ValueError):
             self.module.warn(BadStrWarning())
 
+    def test_simplefilter_invalid_category(self):
+        class MyWarningClass(Warning):
+            pass
+
+        class NonWarningSubclass:
+            pass
+
+        msg_regex = 'category must be a Warning subclass, not (.*)'
+
+        with self.assertRaisesRegex(TypeError, msg_regex):
+            self.module.simplefilter('always', '')
+
+        with self.assertRaisesRegex(TypeError, msg_regex):
+            self.module.simplefilter('always', NonWarningSubclass)
+
+        with self.assertRaisesRegex(TypeError, msg_regex):
+            self.module.simplefilter('always', MyWarningClass())
+
 
 class CWarnTests(WarnTests, unittest.TestCase):
     module = c_warnings
 
     # As an early adopter, we sanity check the
     # test.support.import_fresh_module utility function
     def test_accelerated(self):
         self.assertFalse(original_warnings is self.module)
diff --git a/Lib/warnings.py b/Lib/warnings.py
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -62,16 +62,19 @@ def simplefilter(action, category=Warnin
     'category' -- a class that the warning must be a subclass of
     'lineno' -- an integer line number, 0 matches all warnings
     'append' -- if true, append to the list of filters
     """
     assert action in ("error", "ignore", "always", "default", "module",
                       "once"), "invalid action: %r" % (action,)
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
+    if not (isinstance(category, type) and issubclass(category, Warning)):
+        raise TypeError('category must be a Warning subclass, '
+                        'not {!r}'.format(category))
     item = (action, None, category, None, lineno)
     if append:
         filters.append(item)
     else:
         filters.insert(0, item)
 
 def resetwarnings():
     """Clear the list of warning filters, so that no filters are active."""