diff --git a/doc/requirements.txt b/doc/requirements.txt
index dc85676c..66859e6e 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,5 +1,7 @@
 setuptools_scm
-sphinx>=2.1.0
+# sphinxcontrib-wavedrom isn't yet ready for Sphinx 4. Relax this requirements
+# once https://github.com/bavovanachte/sphinx-wavedrom/pull/32 is released.
+sphinx~=3.0
 sphinx_rtd_theme
 sphinxcontrib-wavedrom
 wavedrom>=1.9.0rc1