diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 69562ba351..f2816a0395 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -13,6 +13,8 @@ on:
 
 jobs:
   Build:
+    # don't schedule nightlies on forks
+    if: github.event_name != 'schedule' || github.repository == 'leanprover/lean4'
     runs-on: ${{ matrix.os }}
     defaults:
       run: