diff --git a/packages/vitest/src/node/plugins/runnerTransform.ts b/packages/vitest/src/node/plugins/runnerTransform.ts
index 017bac54f..e302d5388 100644
--- a/packages/vitest/src/node/plugins/runnerTransform.ts
+++ b/packages/vitest/src/node/plugins/runnerTransform.ts
@@ -145,6 +145,12 @@ export function ModuleRunnerTransform(): VitePlugin {
           // in both SSR and Client environments
           environment.resolve.noExternal = true
 
+          // Workaround `noExternal` merging bug on Vite 6
+          // https://github.com/vitejs/vite/pull/20502
+          if (name === 'ssr') {
+            delete config.ssr?.noExternal
+          }
+
           if (name === '__vitest_vm__' || name === '__vitest__') {
             continue
           }