diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 97ce8cd..56e70e6 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1462,6 +1462,9 @@
     SET_SYS_FROM_STRING("subversion",
                          Py_BuildValue("(ssz)", "CPython", branch,
                                       svn_revision));
+    SET_SYS_FROM_STRING("_mercurial",
+                        Py_BuildValue("(szz)", "CPython", _Py_hgidentifier(),
+                                      _Py_hgversion()));
     SET_SYS_FROM_STRING("dont_write_bytecode",
                          PyBool_FromLong(Py_DontWriteBytecodeFlag));
     SET_SYS_FROM_STRING("api_version",