diff --git a/pyproject.toml b/pyproject.toml
index 4a6ef99..8ae2e20 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -33,6 +33,11 @@ dependencies = [
     "numpy >=1.15",
 ]
 
+[project.optional-dependencies]
+derivations = [
+    "sympy",
+]
+
 [tool.setuptools.packages.find]
 include = ["transforms3d*"]