diff --git a/doc/rtd_environment.yml b/doc/rtd_environment.yml
index d1bbf4a1a..457289681 100644
--- a/doc/rtd_environment.yml
+++ b/doc/rtd_environment.yml
@@ -7,7 +7,7 @@ dependencies:
   - ipykernel
   - ipywidgets
   # documentation
-  - sphinx
+  - sphinx=4.0
   - jinja2=2
   - m2r2
   - sphinxcontrib-napoleon