diff --git a/lib/matplotlib/tests/__init__.py b/lib/matplotlib/tests/__init__.py
index b31594ee23ef..b15f7b0256d1 100644
--- a/lib/matplotlib/tests/__init__.py
+++ b/lib/matplotlib/tests/__init__.py
@@ -6,6 +6,7 @@
 import difflib
 import os
 
+from matplotlib import cbook
 from matplotlib.testing import setup
 
 
@@ -19,6 +20,7 @@
         'test data.')
 
 
+@cbook.deprecated("2.1")
 def assert_str_equal(reference_str, test_str,
                      format_str=('String {str1} and {str2} do not '
                                  'match:\n{differences}')):