diff --git a/src/tox/constants.py b/src/tox/constants.py
index 8dcc86b..1c90e47 100644
--- a/src/tox/constants.py
+++ b/src/tox/constants.py
@@ -30,7 +30,7 @@ def _construct_default_factors(cpython_versions, pypy_versions, other_interprete
 
 class PYTHON:
     PY_FACTORS_RE = re.compile("^(?!py$)(py|pypy|jython)([2-9][0-9]?)?$")
-    CPYTHON_VERSION_TUPLES = [(2, 7), (3, 4), (3, 5), (3, 6), (3, 7), (3, 8)]
+    CPYTHON_VERSION_TUPLES = [(2, 7), (3, 4), (3, 5), (3, 6), (3, 7), (3, 8), (3, 9)]
     PYPY_VERSION_TUPLES = [(2, 7), (3, 5)]
     OTHER_PYTHON_INTERPRETERS = ["jython"]
     DEFAULT_FACTORS = _construct_default_factors(