diff --git a/tests/validate.sh b/tests/validate.sh
index 910e6e3..319fbbd 100755
--- a/tests/validate.sh
+++ b/tests/validate.sh
@@ -20,4 +20,9 @@ for file in "${required_files[@]}"; do
   }
 done
 
+if grep -Eq 'default_branch:[[:space:]]*master\b' workspace.yaml; then
+  printf 'workspace.yaml must not use master as a default branch.\n' >&2
+  exit 1
+fi
+
 printf 'Repository baseline is present.\n'
diff --git a/workspace.yaml b/workspace.yaml
index e43a326..3701b5c 100644
--- a/workspace.yaml
+++ b/workspace.yaml
@@ -6,7 +6,7 @@ workspace:
 repos:
   - name: base
     url: git@github.com:basefoundry/base.git
-    default_branch: master
+    default_branch: main
     required: true
 
   - name: base-workspace