diff --git a/.changeset/ninety-stingrays-double.md b/.changeset/ninety-stingrays-double.md
new file mode 100644
index 0000000000..bf0ceca1b5
--- /dev/null
+++ b/.changeset/ninety-stingrays-double.md
@@ -0,0 +1,5 @@
+---
+"@blitzjs/auth": patch
+---
+
+fix: Overriden custom cookies used inside `withBlitzAuth`
diff --git a/packages/blitz-auth/src/server/auth-sessions.ts b/packages/blitz-auth/src/server/auth-sessions.ts
index 6fd512bd00..71259fc659 100644
--- a/packages/blitz-auth/src/server/auth-sessions.ts
+++ b/packages/blitz-auth/src/server/auth-sessions.ts
@@ -466,7 +466,7 @@ export class SessionContextClass implements SessionContext {
     }
     const cookieHeaders = this._headers.get("set-cookie")
     if (response instanceof Response) {
-      response.headers.set("Set-Cookie", cookieHeaders!)
+      response.headers.append("Set-Cookie", cookieHeaders!)
     } else {
       response.setHeader("Set-Cookie", splitCookiesString(cookieHeaders!))
     }