diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 7f87b438ea5..6e4f0484a4d 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -1223,6 +1223,14 @@ class UnicodeTest(
         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 b8959df4e9a..3a9a64186a4 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -9,6 +9,11 @@ What's New in Python 3.1.3?
 
 *Release date: 20XX-XX-XX*
 
+Core and Builtins
+-----------------
+
+- Issue #1583863: An str subclass can now override the __str__ method
+
 Library
 -------
 
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 5fd23729b6a..2ccec90c089 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -9308,7 +9308,7 @@ 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);
                 }