diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index 8086bf9b274..881de6ffa5b 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -263,6 +263,10 @@ class TestCase(object):
 
     maxDiff = 80*8
 
+    # If a string is longer than _diffThreshold, use normal comparison instead
+    # of difflib.  See #11763.
+    _diffThreshold = 2**16
+
     # Attribute used by TestSuite for classSetUp
 
     _classSetupFailed = False
@@ -1006,6 +1010,10 @@ class TestCase(object):
         self.assertIsInstance(second, str, 'Second argument is not a string')
 
         if first != second:
+            # don't use difflib if the strings are too long
+            if (len(first) > self._diffThreshold or
+                len(second) > self._diffThreshold):
+                self._baseAssertEqual(first, second, msg)
             firstlines = first.splitlines(True)
             secondlines = second.splitlines(True)
             if len(firstlines) == 1 and first.strip('\r\n') == first:
diff --git a/Lib/unittest/test/test_case.py b/Lib/unittest/test/test_case.py
index 82931c012fd..147a4da3a30 100644
--- a/Lib/unittest/test/test_case.py
+++ b/Lib/unittest/test/test_case.py
@@ -720,6 +720,42 @@ class Test_TestCase(unittest.TestCase, TestEquality, TestHashing):
         else:
             self.fail('assertMultiLineEqual did not fail')
 
+    def testAssertEqual_diffThreshold(self):
+        # check threshold value
+        self.assertEqual(self._diffThreshold, 2**16)
+        # disable madDiff to get diff markers
+        self.maxDiff = None
+
+        # set a lower threshold value and add a cleanup to restore it
+        old_threshold = self._diffThreshold
+        self._diffThreshold = 2**8
+        self.addCleanup(lambda: setattr(self, '_diffThreshold', old_threshold))
+
+        # under the threshold: diff marker (^) in error message
+        s = 'x' * (2**7)
+        with self.assertRaises(self.failureException) as cm:
+            self.assertEqual(s + 'a', s + 'b')
+        self.assertIn('^', str(cm.exception))
+        self.assertEqual(s + 'a', s + 'a')
+
+        # over the threshold: diff not used and marker (^) not in error message
+        s = 'x' * (2**9)
+        # if the path that uses difflib is taken, _truncateMessage will be
+        # called -- replace it with explodingTruncation to verify that this
+        # doesn't happen
+        def explodingTruncation(message, diff):
+            raise SystemError('this should not be raised')
+        old_truncate = self._truncateMessage
+        self._truncateMessage = explodingTruncation
+        self.addCleanup(lambda: setattr(self, '_truncateMessage', old_truncate))
+
+        s1, s2 = s + 'a', s + 'b'
+        with self.assertRaises(self.failureException) as cm:
+            self.assertEqual(s1, s2)
+        self.assertNotIn('^', str(cm.exception))
+        self.assertEqual(str(cm.exception), '%r != %r' % (s1, s2))
+        self.assertEqual(s + 'a', s + 'a')
+
     def testAssertCountEqual(self):
         a = object()
         self.assertCountEqual([1, 2, 3], [3, 2, 1])
diff --git a/Misc/NEWS b/Misc/NEWS
index e993aceef03..364e1f9b74a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -123,6 +123,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #11763: don't use difflib in TestCase.assertMultiLineEqual if the
+  strings are too long.
+
 - Issue #11236: getpass.getpass responds to ctrl-c or ctrl-z on terminal.
 
 - Issue #11856: Speed up parsing of JSON numbers.