diff --git a/lib/spack/docs/conf.py b/lib/spack/docs/conf.py
index 5e2795cf49c..2b58223e1dc 100644
--- a/lib/spack/docs/conf.py
+++ b/lib/spack/docs/conf.py
@@ -107,6 +107,7 @@ def setup(sphinx):
     'sphinx.ext.intersphinx',
     'sphinx.ext.napoleon',
     'sphinx.ext.todo',
+    'sphinx.ext.viewcode',
     'sphinxcontrib.programoutput',
 ]