diff --git a/docs/requirements.txt b/docs/requirements.txt
index 169a365fa..14e6e5c49 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -13,6 +13,10 @@ myst-parser[sphinx]
 # The next packages are for notebooks
 matplotlib
 sklearn
+# RTD defaults to an older version of pygments which is incompatible with the
+# nbsphinx/jupyter versions that gets installed. This can be removed if/when RTD
+# defaults to a newer version.
+pygments==2.4.1
 # For CI tests.
 pytest
 pytest-xdist