diff -r 58c8cae6c61a Doc/library/cmath.rst
--- a/Doc/library/cmath.rst	Fri Aug 12 13:21:05 2016 +0000
+++ b/Doc/library/cmath.rst	Sun Aug 14 21:13:26 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 58c8cae6c61a Doc/library/math.rst
--- a/Doc/library/math.rst	Fri Aug 12 13:21:05 2016 +0000
+++ b/Doc/library/math.rst	Sun Aug 14 21:13:26 2016 -0700
@@ -426,6 +426,13 @@
 
    The mathematical constant e = 2.718281..., to available precision.
 
+.. data:: tau
+
+   The mathematical constant τ = 6.283185..., to available precision.
+   Tau is a circle constant equal to 2π, the ratio of a circle's circumference to
+   its radius. To learn more about Tau, check out Vi Hart's video `Pi is (still)
+   Wrong <https://www.youtube.com/watch?v=jG7vhMMXagQ>`_, and start celebrating
+   `Tau day <http://tauday.com/>`_ by eating twice as much pie!
 
 .. data:: inf
 
diff -r 58c8cae6c61a Include/pymath.h
--- a/Include/pymath.h	Fri Aug 12 13:21:05 2016 +0000
+++ b/Include/pymath.h	Sun Aug 14 21:13:26 2016 -0700
@@ -55,6 +55,12 @@
 #define Py_MATH_E 2.7182818284590452354
 #endif
 
+/* Tau (2pi) to 40 digits, taken from tauday.com/tau-digits. */
+#ifndef Py_MATH_TAU
+#define Py_MATH_TAU 6.2831853071795864769252867665590057683943L
+#endif
+
+
 /* On x86, Py_FORCE_DOUBLE forces a floating-point number out of an x87 FPU
    register and into a 64-bit memory location, rounding from extended
    precision to double precision in the process.  On other platforms it does
diff -r 58c8cae6c61a Lib/test/test_math.py
--- a/Lib/test/test_math.py	Fri Aug 12 13:21:05 2016 +0000
+++ b/Lib/test/test_math.py	Sun Aug 14 21:13:26 2016 -0700
@@ -197,6 +197,7 @@
     def testConstants(self):
         self.ftest('pi', math.pi, 3.1415926)
         self.ftest('e', math.e, 2.7182818)
+        self.assertEqual(math.tau, 2*math.pi)
 
     def testAcos(self):
         self.assertRaises(TypeError, math.acos)
diff -r 58c8cae6c61a Modules/cmathmodule.c
--- a/Modules/cmathmodule.c	Fri Aug 12 13:21:05 2016 +0000
+++ b/Modules/cmathmodule.c	Sun Aug 14 21:13:26 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 */
 
diff -r 58c8cae6c61a Modules/mathmodule.c
--- a/Modules/mathmodule.c	Fri Aug 12 13:21:05 2016 +0000
+++ b/Modules/mathmodule.c	Sun Aug 14 21:13:26 2016 -0700
@@ -2144,6 +2144,7 @@
 
     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 */
     PyModule_AddObject(m, "inf", PyFloat_FromDouble(m_inf()));
 #if !defined(PY_NO_SHORT_FLOAT_REPR) || defined(Py_NAN)
     PyModule_AddObject(m, "nan", PyFloat_FromDouble(m_nan()));