diff --git a/pyproject.toml b/pyproject.toml
index 9ceb5c4..8d4b298 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -15,7 +15,7 @@ requires-python = ">=3.10.0"
 license = { text = "MIT" }
 dependencies = [
     "Deprecated",
-    "gymnasium",
+    "gymnasium<1.0.0",
     "numpy",
     "pandas",
     "pettingzoo",