diff --git a/requirements_dev.txt b/requirements_dev.txt
index f6413e5..22c26bd 100644
--- a/requirements_dev.txt
+++ b/requirements_dev.txt
@@ -15,7 +15,7 @@ wheel==0.29.0
 watchdog==0.8.3
 pytest==3.2.1
 pytest-runner==2.11.1
-tox==2.7.0
+tox==4.21.1
 flake8==3.4.1
 coverage==4.4.1
 Sphinx==1.6.3