diff --git a/.readthedocs.yml b/.readthedocs.yml
index d6ca0316b3..18f81acab2 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -1,17 +1,20 @@
 # Configuration for the Read The Docs (RTD) builds of the documentation.
 # Ref: https://docs.readthedocs.io/en/stable/config-file/v2.html
-# Note python.install.requirements is not currently required, as Sphinx is
-# preinstalled and spelling checks not performed by RTD.
+# The python.install.requirements pins the version of Sphinx used.
 version: 2
 
 build:
   os: ubuntu-20.04
   tools:
-    python: "3.10"
+    python: "3.8"
 
 sphinx:
   configuration: docs/conf.py
 
+python:
+  install:
+    - requirements: docs/requirements.txt
+
 formats:
   - epub
   - pdf
diff --git a/docs/requirements.txt b/docs/requirements.txt
new file mode 100644
index 0000000000..6ea1372680
--- /dev/null
+++ b/docs/requirements.txt
@@ -0,0 +1,3 @@
+pyenchant
+Sphinx>=3.1.0
+sphinxcontrib-spelling