diff --git a/pyproject.toml b/pyproject.toml
index 9de1588..71126dd 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -3,6 +3,7 @@ requires = [
     "setuptools>=64",
     "setuptools_scm>=8",
     "cython>=0.29.16",
+    "cython<3.1.0; python_version<'3.9'",
     "cmake",
     "numpy>=1.25.0; python_version>='3.9'",
     "oldest-supported-numpy; python_version<'3.9'",