diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index b345f7875..5cfd4b94c 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -15,6 +15,8 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3.11"
+  apt_packages:
+    - graphviz   
   jobs:
     pre_build:
       - ford fortran_docs.md