diff --git a/include/swift/Basic/Features.def b/include/swift/Basic/Features.def
index 355360f09e29c..220e93ae986cc 100644
--- a/include/swift/Basic/Features.def
+++ b/include/swift/Basic/Features.def
@@ -465,9 +465,6 @@ EXPERIMENTAL_FEATURE(ImportNonPublicCxxMembers, true)
 // Isolated deinit
 SUPPRESSIBLE_LANGUAGE_FEATURE(IsolatedDeinit, 371, "isolated deinit")
 
-// When a parameter has unspecified isolation, infer it as main actor isolated.
-EXPERIMENTAL_FEATURE(UnspecifiedMeansMainActorIsolated, false)
-
 /// modify/read single-yield coroutines
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(CoroutineAccessors, true)
 
diff --git a/include/swift/Basic/LangOptions.h b/include/swift/Basic/LangOptions.h
index 0ab9297ad68f6..5d9cac773637b 100644
--- a/include/swift/Basic/LangOptions.h
+++ b/include/swift/Basic/LangOptions.h
@@ -96,6 +96,11 @@ namespace swift {
     TaskToThread,
   };
 
+  enum class DefaultIsolation : uint8_t {
+    MainActor,
+    Nonisolated
+  };
+
   /// Describes the code size optimization behavior for code associated with
   /// declarations that are marked unavailable.
   enum class UnavailableDeclOptimization : uint8_t {
@@ -632,6 +637,9 @@ namespace swift {
     /// Disables `DynamicActorIsolation` feature.
     bool DisableDynamicActorIsolation = false;
 
+    /// Defines the default actor isolation.
+    DefaultIsolation DefaultIsolationBehavior = DefaultIsolation::Nonisolated;
+
     /// Whether or not to allow experimental features that are only available
     /// in "production".
 #ifdef NDEBUG
diff --git a/include/swift/Option/Options.td b/include/swift/Option/Options.td
index c16f7b1a533a8..f446f81af66c3 100644
--- a/include/swift/Option/Options.td
+++ b/include/swift/Option/Options.td
@@ -985,6 +985,15 @@ def strict_concurrency : Joined<["-"], "strict-concurrency=">,
            "concurrency model, or 'complete' ('Sendable' and other checking is "
            "enabled for all code in the module)">;
 
+def default_isolation : Separate<["-"], "default-isolation">,
+  Flags<[FrontendOption]>,
+  HelpText<"Specify the default actor isolation: MainActor or nonisolated. "
+  "Defaults to nonisolated.">,
+  MetaVarName<"MainActor|nonisolated">;
+def default_isolation_EQ :  Joined<["-"], "default-isolation=">,
+  Flags<[FrontendOption]>,
+  Alias<default_isolation>;
+
 def enable_experimental_feature :
   Separate<["-"], "enable-experimental-feature">,
   Flags<[FrontendOption, ModuleInterfaceOption]>,
diff --git a/lib/AST/FeatureSet.cpp b/lib/AST/FeatureSet.cpp
index 5a3d5f4616e37..136f05861196b 100644
--- a/lib/AST/FeatureSet.cpp
+++ b/lib/AST/FeatureSet.cpp
@@ -205,7 +205,6 @@ UNINTERESTING_FEATURE(StaticExclusiveOnly)
 UNINTERESTING_FEATURE(ExtractConstantsFromMembers)
 UNINTERESTING_FEATURE(GroupActorErrors)
 UNINTERESTING_FEATURE(SameElementRequirements)
-UNINTERESTING_FEATURE(UnspecifiedMeansMainActorIsolated)
 
 static bool usesFeatureSendingArgsAndResults(Decl *decl) {
   auto isFunctionTypeWithSending = [](Type type) {
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index a47134776d6d1..df67034ce7333 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -969,9 +969,6 @@ static bool ParseEnabledFeatureArgs(LangOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_strict_memory_safety))
     Opts.enableFeature(Feature::StrictMemorySafety);
 
-  if (Opts.hasFeature(Feature::UnspecifiedMeansMainActorIsolated))
-    Opts.enableFeature(Feature::InferIsolatedConformances);
-
   return HadError;
 }
 
@@ -1817,6 +1814,27 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DisableDynamicActorIsolation |=
       Args.hasArg(OPT_disable_dynamic_actor_isolation);
 
+  if (const Arg *A = Args.getLastArg(options::OPT_default_isolation)) {
+    auto behavior =
+        llvm::StringSwitch<std::optional<DefaultIsolation>>(A->getValue())
+            .Case("MainActor", DefaultIsolation::MainActor)
+            .Case("nonisolated", DefaultIsolation::Nonisolated)
+            .Default(std::nullopt);
+
+    if (behavior) {
+      Opts.DefaultIsolationBehavior = *behavior;
+    } else {
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                     A->getAsString(Args), A->getValue());
+      HadError = true;
+    }
+  } else {
+    Opts.DefaultIsolationBehavior = DefaultIsolation::Nonisolated;
+  }
+
+  if (Opts.DefaultIsolationBehavior == DefaultIsolation::MainActor)
+    Opts.enableFeature(Feature::InferIsolatedConformances);
+
 #if !defined(NDEBUG) && SWIFT_ENABLE_EXPERIMENTAL_PARSER_VALIDATION
   /// Enable round trip parsing via the new swift parser unless it is disabled
   /// explicitly. The new Swift parser can have mismatches with C++ parser -
diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index 5a36e479e2d66..359bc2a2362e8 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -5798,7 +5798,7 @@ computeDefaultInferredActorIsolation(ValueDecl *value) {
 
     // If we are required to use main actor... just use that.
     if (!ignoreUnspecifiedMeansMainActorIsolated &&
-        ctx.LangOpts.hasFeature(Feature::UnspecifiedMeansMainActorIsolated))
+        ctx.LangOpts.DefaultIsolationBehavior == DefaultIsolation::MainActor)
       if (auto result =
               globalActorHelper(ctx.getMainActorType()->mapTypeOutOfContext()))
         return *result;
diff --git a/test/Concurrency/Runtime/unspecified_is_main_actor.swift b/test/Concurrency/Runtime/unspecified_is_main_actor.swift
index fe2d312a18ece..51c3a732079c2 100644
--- a/test/Concurrency/Runtime/unspecified_is_main_actor.swift
+++ b/test/Concurrency/Runtime/unspecified_is_main_actor.swift
@@ -1,11 +1,10 @@
-// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -swift-version 6 -g -import-objc-header %S/Inputs/RunOnMainActor.h %import-libdispatch -enable-experimental-feature UnspecifiedMeansMainActorIsolated )
+// RUN: %target-run-simple-swift( -Xfrontend -disable-availability-checking -swift-version 6 -g -import-objc-header %S/Inputs/RunOnMainActor.h %import-libdispatch -Xfrontend -default-isolation=MainActor )
 
 // REQUIRES: executable_test
 // REQUIRES: concurrency
 // REQUIRES: concurrency_runtime
 // REQUIRES: libdispatch
 // REQUIRES: asserts
-// REQUIRES: swift_feature_UnspecifiedMeansMainActorIsolated
 
 // UNSUPPORTED: freestanding
 
diff --git a/test/Concurrency/assume_mainactor.swift b/test/Concurrency/assume_mainactor.swift
index 4064bb7582de8..5ac53cb6bc911 100644
--- a/test/Concurrency/assume_mainactor.swift
+++ b/test/Concurrency/assume_mainactor.swift
@@ -1,10 +1,8 @@
-// RUN: %target-swift-frontend -swift-version 6 -emit-silgen -enable-experimental-feature UnspecifiedMeansMainActorIsolated %s | %FileCheck %s
-// RUN: %target-swift-frontend -swift-version 6 -emit-sil -enable-experimental-feature UnspecifiedMeansMainActorIsolated %s -verify
-
-// REQUIRES: swift_feature_UnspecifiedMeansMainActorIsolated
+// RUN: %target-swift-frontend -swift-version 6 -emit-silgen -default-isolation MainActor %s | %FileCheck %s
+// RUN: %target-swift-frontend -swift-version 6 -emit-sil -default-isolation MainActor %s -verify
 
 // READ THIS! This test is meant to FileCheck the specific isolation when
-// UnspecifiedMeansMainActorIsolated is enabled. Please do not put other types
+// `-default-isolation` is set to `MainActor`. Please do not put other types
 // of tests in here.
 
 class Klass {
diff --git a/test/Concurrency/assume_mainactor_typechecker_errors.swift b/test/Concurrency/assume_mainactor_typechecker_errors.swift
index 6f70c3bfa8579..7ca7f42af09c6 100644
--- a/test/Concurrency/assume_mainactor_typechecker_errors.swift
+++ b/test/Concurrency/assume_mainactor_typechecker_errors.swift
@@ -1,9 +1,7 @@
-// RUN: %target-swift-frontend -swift-version 6 -emit-sil -enable-experimental-feature UnspecifiedMeansMainActorIsolated %s -verify
-
-// REQUIRES: swift_feature_UnspecifiedMeansMainActorIsolated
+// RUN: %target-swift-frontend -swift-version 6 -emit-sil -default-isolation MainActor %s -verify
 
 // READ THIS! This test is meant to check the specific isolation when
-// UnspecifiedMeansMainActorIsolated is enabled in combination with validating
+// `-default-isolation` is set to `MainActor` in combination with validating
 // behavior around explicitly non-Sendable types that trigger type checker
 // specific errors. Please do not put other types of tests in here.
 
diff --git a/test/Concurrency/isolated_conformance_default_actor.swift b/test/Concurrency/isolated_conformance_default_actor.swift
index 11db1aefa69cf..3b30223420731 100644
--- a/test/Concurrency/isolated_conformance_default_actor.swift
+++ b/test/Concurrency/isolated_conformance_default_actor.swift
@@ -1,7 +1,6 @@
-// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 6 -enable-experimental-feature IsolatedConformances -enable-experimental-feature UnspecifiedMeansMainActorIsolated %s
+// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 6 -enable-experimental-feature IsolatedConformances -default-isolation MainActor %s
 
 // REQUIRES: swift_feature_IsolatedConformances
-// REQUIRES: swift_feature_UnspecifiedMeansMainActorIsolated
 // REQUIRES: concurrency
 
 nonisolated
diff --git a/test/Concurrency/swiftsettings_defaultIsolation.swift b/test/Concurrency/swiftsettings_defaultIsolation.swift
index aca191b7cbd37..7ecb695a07722 100644
--- a/test/Concurrency/swiftsettings_defaultIsolation.swift
+++ b/test/Concurrency/swiftsettings_defaultIsolation.swift
@@ -1,11 +1,10 @@
 // RUN: %target-swift-frontend -enable-experimental-feature SwiftSettings -c -verify -swift-version 6 -verify-additional-prefix nonisolated- -disable-availability-checking %s
 // RUN: %target-swift-frontend -enable-experimental-feature SwiftSettings -c -verify -swift-version 6 -verify-additional-prefix main-actor- -disable-availability-checking -DSWIFT_SETTINGS_MAIN_ACTOR %s
-// RUN: %target-swift-frontend -enable-experimental-feature UnspecifiedMeansMainActorIsolated -enable-experimental-feature SwiftSettings -c -verify -swift-version 6 -disable-availability-checking -verify-additional-prefix main-actor- %s
-// RUN: %target-swift-frontend -enable-experimental-feature UnspecifiedMeansMainActorIsolated -enable-experimental-feature SwiftSettings -c -verify -swift-version 6 -disable-availability-checking -verify-additional-prefix nonisolated- -DSWIFT_SETTINGS_NONISOLATED %s
+// RUN: %target-swift-frontend -default-isolation MainActor -enable-experimental-feature SwiftSettings -c -verify -swift-version 6 -disable-availability-checking -verify-additional-prefix main-actor- %s
+// RUN: %target-swift-frontend -default-isolation MainActor -enable-experimental-feature SwiftSettings -c -verify -swift-version 6 -disable-availability-checking -verify-additional-prefix nonisolated- -DSWIFT_SETTINGS_NONISOLATED %s
 
 // REQUIRES: asserts
 // REQUIRES: concurrency
-// REQUIRES: swift_feature_UnspecifiedMeansMainActorIsolated
 // REQUIRES: swift_feature_SwiftSettings
 
 #if SWIFT_SETTINGS_MAIN_ACTOR