diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 1da2d1491cd..4ac07123b86 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1221,6 +1221,14 @@ class UnicodeTest(string_tests.CommonTest,
         self.assertRaises(MemoryError, alloc)
         self.assertRaises(MemoryError, alloc)
 
+    def test_format_subclass(self):
+        class S(str):
+            def __str__(self):
+                return '__str__ overridden'
+        s = S('xxx')
+        self.assertEquals("%s" % s, '__str__ overridden')
+        self.assertEquals("{}".format(s), '__str__ overridden')
+
 
 def test_main():
     support.run_unittest(__name__)
diff --git a/Misc/NEWS b/Misc/NEWS
index cfe45fb6661..8a62cbf4a29 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,8 @@ What's New in Python 3.2 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #1583863: An str subclass can now override the __str__ method
+
 - Issue #8014: Setting a T_UINT or T_PYSSIZET attribute of an object with
   PyMemberDefs could produce an internal error;  raise TypeError instead.
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 3ee90b58ddb..28b8c66295a 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -9124,7 +9124,7 @@ PyObject *PyUnicode_Format(PyObject *format,
             case 's':
             case 'r':
             case 'a':
-                if (PyUnicode_Check(v) && c == 's') {
+                if (PyUnicode_CheckExact(v) && c == 's') {
                     temp = v;
                     Py_INCREF(temp);
                 }