diff --git a/requirements_dev.txt b/requirements_dev.txt
index 861d348..b1c8220 100644
--- a/requirements_dev.txt
+++ b/requirements_dev.txt
@@ -3,7 +3,7 @@ bumpversion==0.5.3
 wheel==0.33.1
 watchdog==0.9.0
 flake8==3.7.7
-tox==3.9.0
+tox==4.21.1
 coverage==4.5.3
 Sphinx==2.0.1
 twine==1.13.0