diff --git a/lib/Sema/CodeSynthesis.cpp b/lib/Sema/CodeSynthesis.cpp
index cdffea876a224..f94584a2a395a 100644
--- a/lib/Sema/CodeSynthesis.cpp
+++ b/lib/Sema/CodeSynthesis.cpp
@@ -1703,9 +1703,14 @@ SynthesizeDefaultInitRequest::evaluate(Evaluator &evaluator,
       ImplicitConstructorKind::DefaultDistributedActor :
       ImplicitConstructorKind::Default;
   if (auto ctor = createImplicitConstructor(decl, ctorKind, ctx)) {
+    if (auto isolation = decl->getDefaultActorIsolation()) {
+    /// Ensure the synthesized default initializer inherits the class's
+    /// default actor isolation (e.g., @MainActor) to avoid
+    /// override-isolation mismatches in subclasses.
+      ctor->setActorIsolation(isolation);
+    }
     // Add the constructor.
     decl->addMember(ctor);
-
     // Lazily synthesize an empty body for the default constructor.
     ctor->setBodySynthesizer(synthesizeSingleReturnFunctionBody);
     return ctor;
diff --git a/test/Concurrency/default_actor_isolation_initializer.swift b/test/Concurrency/default_actor_isolation_initializer.swift
new file mode 100644
index 0000000000000..63403face1a6d
--- /dev/null
+++ b/test/Concurrency/default_actor_isolation_initializer.swift
@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift -enable-upcoming-feature GlobalActorIsMainActor -enable-upcoming-feature RegionBasedIsolation
+// expected-no-diagnostics
+
+@MainActor
+class Thing {}
+
+class SubThing: Thing {}