diff -r d9e83e426565 Lib/json/__init__.py
--- a/Lib/json/__init__.py	Fri Jun 26 02:50:21 2015 -0700
+++ b/Lib/json/__init__.py	Sat Jun 27 11:06:17 2015 -0400
@@ -153,7 +153,8 @@
     of obj or raise TypeError. The default simply raises TypeError.
 
     If *sort_keys* is ``True`` (default: ``False``), then the output of
-    dictionaries will be sorted by key.
+    dictionaries will be sorted by key.  If *sort_keys* is a callable,
+    it will be used as the sort key function.
 
     To use a custom ``JSONEncoder`` subclass (e.g. one that overrides the
     ``.default()`` method to serialize additional types), specify it with
diff -r d9e83e426565 Lib/json/encoder.py
--- a/Lib/json/encoder.py	Fri Jun 26 02:50:21 2015 -0700
+++ b/Lib/json/encoder.py	Sat Jun 27 11:06:17 2015 -0400
@@ -128,6 +128,7 @@
         If sort_keys is true, then the output of dictionaries will be
         sorted by key; this is useful for regression tests to ensure
         that JSON serializations can be compared on a day-to-day basis.
+        If sort_keys is a callable, it will be used as the sort key function.
 
         If indent is a non-negative integer, then JSON array
         elements and object members will be pretty-printed with that
@@ -244,7 +245,8 @@
 
 
         if (_one_shot and c_make_encoder is not None
-                and self.indent is None):
+                and self.indent is None
+                and not callable(self.sort_keys)):
             _iterencode = c_make_encoder(
                 markers, self.default, _encoder, self.indent,
                 self.key_separator, self.item_separator, self.sort_keys,
@@ -349,7 +351,11 @@
             item_separator = _item_separator
         first = True
         if _sort_keys:
-            items = sorted(dct.items(), key=lambda kv: kv[0])
+            if callable(_sort_keys):
+                sorter = lambda kv: _sort_keys(kv[0])
+            else:
+                sorter = lambda kv: kv[0]
+            items = sorted(dct.items(), key=sorter)
         else:
             items = dct.items()
         for key, value in items:
diff -r d9e83e426565 Lib/test/test_json/test_sort.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/test/test_json/test_sort.py	Sat Jun 27 11:06:17 2015 -0400
@@ -0,0 +1,17 @@
+from io import StringIO
+from test.test_json import PyTest, CTest
+
+
+class TestSort:
+    # Issue 24518
+    def test_callable_sort_keys(self):
+        data = {1: 1, 2: 2, 3: 3, 4: 4, 5: 5}
+        is_odd = lambda n: n % 2
+        sio = StringIO()
+        self.json.dump(data, sio, sort_keys=is_odd)
+        result = sio.getvalue()
+        self.assertTrue(result.find("2") < result.find("1"))
+        self.assertTrue(result.find("4") < result.find("3"))
+
+class TestPyIndent(TestSort, PyTest): pass
+class TestCIndent(TestSort, CTest): pass