diff -r 6d278f426417 Lib/json/encoder.py
--- a/Lib/json/encoder.py	Fri Jul 05 18:05:29 2013 -1000
+++ b/Lib/json/encoder.py	Sun Jul 07 09:51:25 2013 +0200
@@ -1,6 +1,7 @@
 """Implementation of JSONEncoder
 """
 import re
+import decimal
 
 try:
     from _json import encode_basestring_ascii as c_encode_basestring_ascii
@@ -260,6 +261,7 @@
         list=list,
         str=str,
         tuple=tuple,
+        Decimal=decimal.Decimal,
     ):
 
     if _indent is not None and not isinstance(_indent, str):
@@ -301,6 +303,8 @@
                 yield buf + str(value)
             elif isinstance(value, float):
                 yield buf + _floatstr(value)
+            elif isinstance(value, Decimal):
+                yield buf + str(value)
             else:
                 yield buf
                 if isinstance(value, (list, tuple)):
@@ -377,6 +381,8 @@
                 yield str(value)
             elif isinstance(value, float):
                 yield _floatstr(value)
+            elif isinstance(value, decimal.Decimal):
+                yield str(value)
             else:
                 if isinstance(value, (list, tuple)):
                     chunks = _iterencode_list(value, _current_indent_level)
@@ -405,6 +411,8 @@
             yield str(o)
         elif isinstance(o, float):
             yield _floatstr(o)
+        elif isinstance(o, decimal.Decimal):
+            yield str(o)
         elif isinstance(o, (list, tuple)):
             yield from _iterencode_list(o, _current_indent_level)
         elif isinstance(o, dict):
diff -r 6d278f426417 Lib/test/json_tests/test_decimal.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Lib/test/json_tests/test_decimal.py	Sun Jul 07 09:51:25 2013 +0200
@@ -0,0 +1,34 @@
+"""
+Unittest module to check for correct handling of Decimals in JSON encoding
+"""
+import math, json
+from decimal import Decimal
+from test.json_tests import PyTest, CTest
+
+
+class TestDecimal:
+    """Test that Decimals are correctly encoded into JSON"""
+    def test_decimals(self):
+        """Check for a few Decimal to be rendered correctly into JSON"""
+        for num in [Decimal('1617161771.7650001'), Decimal(str(math.pi)), 
+                    Decimal('1'), Decimal('-55555555555555555555.555555555')]:
+            self.assertEqual(Decimal(self.dumps(num)), num)
+            self.assertEqual(self.loads(self.dumps(num), parse_float=Decimal), 
+                             num)
+
+    def test_decimal_in_a_list(self):
+        """Check that a Decimal within a list is handled properly"""
+        l = [Decimal('0.2')]
+        self.assertEqual(json.dumps(l), '[0.2]')
+        self.assertEqual(self.loads(self.dumps(l), parse_float=Decimal), l)
+
+    def test_decimal_in_a_dict(self):
+        """Check that a Decimal within a dict (as key and value) 
+           is handled properly.
+        """
+        d = {Decimal('4.4'): Decimal('0.2')}
+        self.assertEqual(json.dumps(d), '{"4.4": 0.2}')
+
+
+class TestPyFloat(TestDecimal, PyTest): pass
+class TestCFloat(TestDecimal, CTest): pass
diff -r 6d278f426417 Modules/_json.c
--- a/Modules/_json.c	Fri Jul 05 18:05:29 2013 -1000
+++ b/Modules/_json.c	Sun Jul 07 09:51:25 2013 +0200
@@ -13,6 +13,7 @@
 #define PyEncoder_Check(op) PyObject_TypeCheck(op, &PyEncoderType)
 #define PyEncoder_CheckExact(op) (Py_TYPE(op) == &PyEncoderType)
 
+static PyTypeObject *PyDecimalType;
 static PyTypeObject PyScannerType;
 static PyTypeObject PyEncoderType;
 
@@ -117,6 +118,8 @@
 encoder_encode_string(PyEncoderObject *s, PyObject *obj);
 static PyObject *
 encoder_encode_float(PyEncoderObject *s, PyObject *obj);
+static PyObject *
+encoder_encode_decimal(PyEncoderObject *s, PyObject *obj);
 
 #define S_CHAR(c) (c >= ' ' && c <= '~' && c != '\\' && c != '"')
 #define IS_WHITESPACE(c) (((c) == ' ') || ((c) == '\t') || ((c) == '\n') || ((c) == '\r'))
@@ -1326,6 +1329,14 @@
 }
 
 static PyObject *
+encoder_encode_decimal(PyEncoderObject *s, PyObject *obj)
+{
+    /* Return the JSON representation of a Decimal object */
+    /* Use a better Decimal formatting here? */
+    return PyObject_Str(obj);
+}
+
+static PyObject *
 encoder_encode_string(PyEncoderObject *s, PyObject *obj)
 {
     /* Return the JSON representation of a string */
@@ -1391,6 +1402,12 @@
         Py_LeaveRecursiveCall();
         return rv;
     }
+    else if (PyObject_IsInstance(obj, (PyObject*)PyDecimalType)) {
+        PyObject *encoded = encoder_encode_decimal(s, obj);
+        if (encoded == NULL)
+            return -1;
+        return _steal_accumulate(acc, encoded);
+    }
     else {
         PyObject *ident = NULL;
         if (s->markers != Py_None) {
@@ -1543,6 +1560,11 @@
             if (kstr == NULL)
                 goto bail;
         }
+        else if (PyObject_IsInstance(key, (PyObject*)PyDecimalType)) {
+            kstr = encoder_encode_decimal(s, key);
+            if (kstr == NULL)
+                goto bail;
+        }
         else if (key == Py_True || key == Py_False || key == Py_None) {
                         /* This must come before the PyLong_Check because
                            True and False are also 1 and 0.*/
@@ -1821,6 +1843,7 @@
 PyObject*
 PyInit__json(void)
 {
+    PyObject *decimal;
     PyObject *m = PyModule_Create(&jsonmodule);
     if (!m)
         return NULL;
@@ -1840,6 +1863,14 @@
         Py_DECREF((PyObject*)&PyEncoderType);
         goto fail;
     }
+    /* Get a reference to the Decimal type and store it in static PyDecimalType
+       declared above */
+    decimal = PyImport_ImportModule("decimal");
+    if (!decimal)
+        goto fail;
+    PyDecimalType = (PyTypeObject*)PyObject_GetAttrString(decimal, "Decimal");
+    Py_DECREF(decimal);
+    Py_INCREF(PyDecimalType);
     return m;
   fail:
     Py_DECREF(m);