diff --git a/Lib/test/test_warnings/__init__.py b/Lib/test/test_warnings/__init__.py
index 4b1b4e193cb165..e2becb74a1c1af 100644
--- a/Lib/test/test_warnings/__init__.py
+++ b/Lib/test/test_warnings/__init__.py
@@ -342,6 +342,13 @@ def test_filterwarnings_duplicate_filters(self):
                 "the beginning of list"
             )
 
+    def test_simplefilter_invalid_category(self):
+        with original_warnings.catch_warnings(record=True, module=self.module):
+            invalid_category = 'spam'
+            with self.assertRaises(AssertionError) as cm:
+                self.module.simplefilter("once", invalid_category)
+            self.assertEqual(str(cm.exception), "category must be a class")
+
     def test_simplefilter_duplicate_filters(self):
         with original_warnings.catch_warnings(module=self.module):
             self.module.resetwarnings()
diff --git a/Lib/warnings.py b/Lib/warnings.py
index 691ccddfa450ad..5df7c8c29a7cc9 100644
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -176,6 +176,7 @@ def simplefilter(action, category=Warning, lineno=0, append=False):
                       "once"), "invalid action: %r" % (action,)
     assert isinstance(lineno, int) and lineno >= 0, \
            "lineno must be an int >= 0"
+    assert isinstance(category, type), "category must be a class"
     _add_filter(action, None, category, None, lineno, append=append)
 
 def _add_filter(*item, append):
diff --git a/Misc/NEWS.d/next/Library/2021-06-12-20-21-40.bpo-16845.SqR6Ik.rst b/Misc/NEWS.d/next/Library/2021-06-12-20-21-40.bpo-16845.SqR6Ik.rst
new file mode 100644
index 00000000000000..8678300e12a5db
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-06-12-20-21-40.bpo-16845.SqR6Ik.rst
@@ -0,0 +1 @@
+Add category type check to warnings.simplefilter.