diff --git a/requirements_dev.txt b/requirements_dev.txt
index ab43b8e..0961366 100644
--- a/requirements_dev.txt
+++ b/requirements_dev.txt
@@ -3,7 +3,7 @@ bumpversion==0.6.0
 wheel==0.36.2
 watchdog==1.0.2
 flake8==3.8.4
-tox==3.20.1
+tox==3.21.1
 coverage==5.3.1
 Sphinx==3.4.1
 cryptography==3.3.1