diff --git a/pyproject.toml b/pyproject.toml
index e27c1736..416112a1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -46,7 +46,7 @@ dev = [
   # Setup.
   "pre-commit==3.8.0 ; python_version>='3.9'",
   "tox==3.27.1 ; python_version<'3.8'",
-  "tox==4.18.0 ; python_version>='3.8'",
+  "tox==4.21.0 ; python_version>='3.8'",
   # Testing framework.
   "pytest==6.1.2 ; python_version<'3.8'",
   "pytest==8.3.2 ; python_version>='3.8'",