diff --git a/CHANGELOG b/CHANGELOG
index b3e4010ee..83f63601c 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -13,6 +13,8 @@ Unreleased
   at the beginning of strings and even that is deprecated.  Use "not" instead.
   This should allow to pick parametrized tests where "-" appeared in the parameter.
 
+- fix issue604: Escape % character in the assertion message.
+
 2.6.3
 -----------
 
diff --git a/_pytest/assertion/rewrite.py b/_pytest/assertion/rewrite.py
index 18df7791e..57edcd487 100644
--- a/_pytest/assertion/rewrite.py
+++ b/_pytest/assertion/rewrite.py
@@ -373,7 +373,7 @@ def _format_assertmsg(obj):
         t = py.builtin.text
     else:
         t = py.builtin.bytes
-    s = s.replace(t("\n"), t("\n~"))
+    s = s.replace(t("\n"), t("\n~")).replace(t("%"), t("%%"))
     if is_repr:
         s = s.replace(t("\\n"), t("\n~"))
     return s
diff --git a/testing/test_assertrewrite.py b/testing/test_assertrewrite.py
index 9d69b5f49..225608281 100644
--- a/testing/test_assertrewrite.py
+++ b/testing/test_assertrewrite.py
@@ -172,6 +172,18 @@ class TestAssertionRewrite:
             "*assert 1 == 2*",
         ])
 
+    def test_assertion_message_escape(self, testdir):
+        testdir.makepyfile("""
+            def test_foo():
+                assert 1 == 2, 'To be escaped: %'
+        """)
+        result = testdir.runpytest()
+        assert result.ret == 1
+        result.stdout.fnmatch_lines([
+            "*AssertionError: To be escaped: %",
+            "*assert 1 == 2",
+        ])
+
     def test_boolop(self):
         def f():
             f = g = False