diff --git a/.readthedocs.yml b/.readthedocs.yml
index 9ab3d482..db303bdf 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -21,4 +21,4 @@ python:
   - method: pip
     path: .
     extra_requirements:
-      - sphinx-rtd-theme
+      - docs   # pip install .[docs]
diff --git a/setup.py b/setup.py
index f5eca1fc..eb086dd5 100644
--- a/setup.py
+++ b/setup.py
@@ -23,8 +23,14 @@
     'pytest_timeout',
 ]
 
+docs_require = [
+    'sphinx',
+    'sphinx-rtd-theme',
+]
+
 extras_require = {
     'test': tests_require,
+    'docs': docs_require,
 }
 
 if platform.python_implementation() != 'PyPy':
diff --git a/tox.ini b/tox.ini
index 2252504c..d215e606 100644
--- a/tox.ini
+++ b/tox.ini
@@ -49,8 +49,8 @@ commands =
 
 [testenv:docs]
 deps =
-  Sphinx
-  sphinx_rtd_theme
+  sphinx
+  sphinx-rtd-theme
 changedir = {toxinidir}/docs
 commands =
   sphinx-build -b html -d {envtmpdir}/doctrees . {envtmpdir}/html