diff --git a/wptrunner/testloader.py b/wptrunner/testloader.py
index c431c840d1208f..2e5f3a51a45e4b 100644
--- a/wptrunner/testloader.py
+++ b/wptrunner/testloader.py
@@ -496,7 +496,7 @@ def load_metadata(self, test_manifest, metadata_path, test_path):
     def iter_tests(self):
         manifest_items = []
 
-        for manifest in self.manifests.keys():
+        for manifest in sorted(self.manifests.keys()):
             manifest_iter = iterfilter(self.manifest_filters,
                                        manifest.itertypes(*self.test_types))
             manifest_items.extend(manifest_iter)