diff --git a/src/node_realm.h b/src/node_realm.h
index 9ad38be761d..63a23013cb1 100644
--- a/src/node_realm.h
+++ b/src/node_realm.h
@@ -209,7 +209,7 @@ class Realm : public MemoryRetainer {
   CppgcWrapperList cppgc_wrapper_list_;
 };
 
-class PrincipalRealm : public Realm {
+class PrincipalRealm final : public Realm {
  public:
   PrincipalRealm(Environment* env,
                  v8::Local<v8::Context> context,
diff --git a/src/node_shadow_realm.h b/src/node_shadow_realm.h
index c2a6f2c8cb8..65844349f7a 100644
--- a/src/node_shadow_realm.h
+++ b/src/node_shadow_realm.h
@@ -9,7 +9,7 @@
 namespace node {
 namespace shadow_realm {
 
-class ShadowRealm : public Realm {
+class ShadowRealm final : public Realm {
  public:
   static ShadowRealm* New(Environment* env);