diff --git a/pyproject.toml b/pyproject.toml
index 01562cefb..70d00626c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -39,6 +39,14 @@ dependencies = [
 ]
 dynamic = ["version"]
 
+[project.optional-dependencies]
+tests = [
+  "Cython",
+  "packaging",
+  "pytest",
+]
+
+
 [project.readme]
 text = """\
 netCDF version 4 has many features not found in earlier versions of the library,