diff --git a/tornado/httputil.py b/tornado/httputil.py
index 13a95d909..74dfb87f1 100644
--- a/tornado/httputil.py
+++ b/tornado/httputil.py
@@ -345,7 +345,8 @@ def __contains__(self, name: object) -> bool:
         # in __getitem__ when it's not needed.
         if not isinstance(name, str):
             return False
-        return name in self._as_list
+        norm_name = _normalize_header(name)
+        return norm_name in self._as_list
 
     def __getitem__(self, name: str) -> str:
         header = _normalize_header(name)
diff --git a/tornado/test/httputil_test.py b/tornado/test/httputil_test.py
index 7fa528cdc..9c7ee093f 100644
--- a/tornado/test/httputil_test.py
+++ b/tornado/test/httputil_test.py
@@ -329,6 +329,9 @@ def test_multi_line(self):
             sorted(list(headers.get_all())),
             [("Asdf", "qwer zxcv"), ("Foo", "bar baz"), ("Foo", "even more lines")],
         )
+        # Verify case insensitivity in-operator
+        self.assertTrue("asdf" in headers)
+        self.assertTrue("Asdf" in headers)
 
     def test_continuation(self):
         data = "Foo: bar\r\n\tasdf"