diff --git a/setup.py b/setup.py
index 9be62d95..1fdf14d2 100644
--- a/setup.py
+++ b/setup.py
@@ -138,7 +138,9 @@ def read(*rnames):
       install_requires=['setuptools'],
       python_requires='>=3.7',
       extras_require={
-          'docs': ['Sphinx', 'repoze.sphinx.autointerface'],
+          'docs': ['Sphinx',
+                   'repoze.sphinx.autointerface',
+                   'sphinx_rtd_theme'],
           'test': tests_require,
           'testing': testing_extras,
       },