diff -r c839ec888e71 Doc/library/cmath.rst
--- a/Doc/library/cmath.rst	Wed Aug 10 21:03:23 2016 -0700
+++ b/Doc/library/cmath.rst	Thu Aug 11 20:27:20 2016 -0700
@@ -253,6 +253,10 @@
 
    The mathematical constant *e*, as a float.
 
+.. data:: tau
+   
+   The mathematical constant *τ*, as a float.
+
 .. index:: module: math
 
 Note that the selection of functions is similar, but not identical, to that in
diff -r c839ec888e71 Doc/library/math.rst
--- a/Doc/library/math.rst	Wed Aug 10 21:03:23 2016 -0700
+++ b/Doc/library/math.rst	Thu Aug 11 20:27:20 2016 -0700
@@ -426,6 +426,9 @@
 
    The mathematical constant e = 2.718281..., to available precision.
 
+.. data:: tau
+
+   The mathematical constant τ = 6.283185..., to available precision.
 
 .. data:: inf
 
diff -r c839ec888e71 Lib/test/test_math.py
--- a/Lib/test/test_math.py	Wed Aug 10 21:03:23 2016 -0700
+++ b/Lib/test/test_math.py	Thu Aug 11 20:27:20 2016 -0700
@@ -197,7 +197,7 @@
     def testConstants(self):
         self.ftest('pi', math.pi, 3.1415926)
         self.ftest('e', math.e, 2.7182818)
-        self.ftest('tau', math.tau, 2*math.pi)
+        self.assertEqual(math.tau, 2*math.pi)
 
     def testAcos(self):
         self.assertRaises(TypeError, math.acos)
diff -r c839ec888e71 Modules/cmathmodule.c
--- a/Modules/cmathmodule.c	Wed Aug 10 21:03:23 2016 -0700
+++ b/Modules/cmathmodule.c	Thu Aug 11 20:27:20 2016 -0700
@@ -1239,6 +1239,7 @@
     PyModule_AddObject(m, "pi",
                        PyFloat_FromDouble(Py_MATH_PI));
     PyModule_AddObject(m, "e", PyFloat_FromDouble(Py_MATH_E));
+    PyModule_AddObject(m, "tau", PyFloat_FromDouble(Py_MATH_TAU)); /* 2pi */
 
     /* initialize special value tables */