diff --git a/tox.ini b/tox.ini
index 648380a98..b8fda8766 100644
--- a/tox.ini
+++ b/tox.ini
@@ -309,13 +309,14 @@ commands = {[testenv:dev-osx]commands}
 
 [testenv:docs]
 description = Build documentation
+# Pin sphinx until sphinxcontrib-mermaid is compatible with 8.1
 deps =
   -rrequirements-test.txt
   jupyter
   linkchecker
   nbsphinx
   pypandoc
-  sphinx
+  sphinx<8.1
   sphinx-rtd-theme
   sphinxcontrib-jquery
   sphinxcontrib-mermaid