diff --git a/packages/flutter_tools/lib/src/commands/generate_localizations.dart b/packages/flutter_tools/lib/src/commands/generate_localizations.dart
index 2a62dfccf31..2800d6253b8 100644
--- a/packages/flutter_tools/lib/src/commands/generate_localizations.dart
+++ b/packages/flutter_tools/lib/src/commands/generate_localizations.dart
@@ -147,6 +147,8 @@ class GenerateLocalizationsCommand extends FlutterCommand {
             'generated as a synthetic package or at a specified directory in '
             'the Flutter project.\n'
             '\n'
+            'DEPRECATED: https://flutter.dev/to/flutter-gen-deprecation.\n'
+            '\n'
             'This flag is set to true by default.\n'
             '\n'
             'When synthetic-package is set to false, it will generate the '
diff --git a/packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart b/packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart
index 655b365fc6c..b5c0e6f1339 100644
--- a/packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart
+++ b/packages/flutter_tools/lib/src/dart/generate_synthetic_packages.dart
@@ -15,10 +15,10 @@ Future<void> generateLocalizationsSyntheticPackage({
   required BuildSystem buildSystem,
   required BuildTargets buildTargets,
 }) async {
-
   final FileSystem fileSystem = environment.fileSystem;
   final File l10nYamlFile = fileSystem.file(
-    fileSystem.path.join(environment.projectDir.path, 'l10n.yaml'));
+    fileSystem.path.join(environment.projectDir.path, 'l10n.yaml'),
+  );
 
   // If pubspec.yaml has generate:true and if l10n.yaml exists in the
   // root project directory, check to see if a synthetic package should
@@ -30,7 +30,7 @@ Future<void> generateLocalizationsSyntheticPackage({
   final YamlNode yamlNode = loadYamlNode(l10nYamlFile.readAsStringSync());
   if (yamlNode.value != null && yamlNode is! YamlMap) {
     throwToolExit(
-      'Expected ${l10nYamlFile.path} to contain a map, instead was $yamlNode'
+      'Expected ${l10nYamlFile.path} to contain a map, instead was $yamlNode',
     );
   }
 
@@ -41,8 +41,7 @@ Future<void> generateLocalizationsSyntheticPackage({
     final Object? value = yamlMap['synthetic-package'];
     if (value is! bool && value != null) {
       throwToolExit(
-        'Expected "synthetic-package" to have a bool value, '
-        'instead was "$value"'
+        'Expected "synthetic-package" to have a bool value, instead was "$value"',
       );
     }
 
@@ -52,8 +51,30 @@ Future<void> generateLocalizationsSyntheticPackage({
     if (isSyntheticL10nPackage == false) {
       return;
     }
+  } else if (!environment.useImplicitPubspecResolution) {
+    // --no-implicit-pubspec-resolution was passed, and synthetic-packages: true was not.
+    return;
   }
 
+  if (!environment.useImplicitPubspecResolution) {
+    throwToolExit(
+      'Cannot generate a synthetic package when --no-implicit-pubspec-resolution is passed.\n'
+      '\n'
+      'Synthetic package output (package:flutter_gen) is deprecated: '
+      'https://flutter.dev/to/flutter-gen-deprecation. If you are seeing this '
+      'message either you have provided --no-implicit-pubspec-resolution, or '
+      'it is the default value (see flutter --verbose --help).',
+    );
+  }
+
+  // Log a warning: synthetic-package: true (or implicit true) is deprecated.
+  environment.logger.printWarning(
+    'Synthetic package output (package:flutter_gen) is deprecated: '
+    'https://flutter.dev/to/flutter-gen-deprecation. In a future release, '
+    'synthetic-package will default to `false` and will later be removed '
+    'entirely.',
+  );
+
   final BuildResult result = await buildSystem.build(
     buildTargets.generateLocalizationsTarget,
     environment,
diff --git a/packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart b/packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart
index fe65d0f2655..f29ca480209 100644
--- a/packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart
+++ b/packages/flutter_tools/test/general.shard/dart/generate_synthetic_packages_test.dart
@@ -267,4 +267,185 @@ void main() {
       throwsToolExit(message: 'to have a bool value, instead was "nonBoolValue"'),
     );
   });
+
+  testWithoutContext('synthetic-package: true (implicit) logs a deprecation warning', () async {
+    // Project directory setup for gen_l10n logic.
+    final MemoryFileSystem fileSystem = MemoryFileSystem.test();
+
+    // Add generate:true to pubspec.yaml.
+    final File pubspecFile = fileSystem.file('pubspec.yaml')..createSync();
+    final String content = pubspecFile.readAsStringSync().replaceFirst(
+      '\nflutter:\n',
+      '\nflutter:\n  generate: true\n',
+    );
+    pubspecFile.writeAsStringSync(content);
+
+    // Create a blank l10n.yaml file.
+    fileSystem.file('l10n.yaml').writeAsStringSync('');
+
+    final BufferLogger mockBufferLogger = BufferLogger.test();
+    final Environment environment = Environment.test(
+      fileSystem.currentDirectory,
+      fileSystem: fileSystem,
+      logger: mockBufferLogger,
+      artifacts: Artifacts.test(),
+      processManager: FakeProcessManager.any(),
+    );
+    final TestBuildSystem buildSystem = TestBuildSystem.all(BuildResult(success: true));
+
+    await generateLocalizationsSyntheticPackage(
+      environment: environment,
+      buildSystem: buildSystem,
+      buildTargets: const BuildTargetsImpl(),
+    );
+
+    expect(
+      mockBufferLogger.warningText,
+      contains('https://flutter.dev/to/flutter-gen-deprecation'),
+    );
+  });
+
+  testWithoutContext('synthetic-package: true (explicit) logs a deprecation warning', () async {
+    // Project directory setup for gen_l10n logic.
+    final MemoryFileSystem fileSystem = MemoryFileSystem.test();
+
+    // Add generate:true to pubspec.yaml.
+    final File pubspecFile = fileSystem.file('pubspec.yaml')..createSync();
+    final String content = pubspecFile.readAsStringSync().replaceFirst(
+      '\nflutter:\n',
+      '\nflutter:\n  generate: true\n',
+    );
+    pubspecFile.writeAsStringSync(content);
+    fileSystem.file('l10n.yaml').writeAsStringSync('synthetic-package: true');
+
+    final BufferLogger mockBufferLogger = BufferLogger.test();
+    final Environment environment = Environment.test(
+      fileSystem.currentDirectory,
+      fileSystem: fileSystem,
+      logger: mockBufferLogger,
+      artifacts: Artifacts.test(),
+      processManager: FakeProcessManager.any(),
+    );
+    final TestBuildSystem buildSystem = TestBuildSystem.all(BuildResult(success: true));
+
+    await generateLocalizationsSyntheticPackage(
+      environment: environment,
+      buildSystem: buildSystem,
+      buildTargets: const BuildTargetsImpl(),
+    );
+
+    expect(
+      mockBufferLogger.warningText,
+      contains('https://flutter.dev/to/flutter-gen-deprecation'),
+    );
+  });
+
+  testWithoutContext('synthetic-package: false has no deprecation warning', () async {
+    // Project directory setup for gen_l10n logic
+    final MemoryFileSystem fileSystem = MemoryFileSystem.test();
+
+    // Add generate:true to pubspec.yaml.
+    final File pubspecFile = fileSystem.file('pubspec.yaml')..createSync();
+    final String content = pubspecFile.readAsStringSync().replaceFirst(
+      '\nflutter:\n',
+      '\nflutter:\n  generate: true\n',
+    );
+    pubspecFile.writeAsStringSync(content);
+    fileSystem.file('l10n.yaml').writeAsStringSync('synthetic-package: false');
+
+    final BufferLogger mockBufferLogger = BufferLogger.test();
+    final Environment environment = Environment.test(
+      fileSystem.currentDirectory,
+      fileSystem: fileSystem,
+      logger: mockBufferLogger,
+      artifacts: Artifacts.test(),
+      processManager: FakeProcessManager.any(),
+    );
+    final TestBuildSystem buildSystem = TestBuildSystem.all(BuildResult(success: true));
+
+    await generateLocalizationsSyntheticPackage(
+      environment: environment,
+      buildSystem: buildSystem,
+      buildTargets: const BuildTargetsImpl(),
+    );
+
+    expect(
+      mockBufferLogger.warningText,
+      isNot(contains('https://flutter.dev/to/flutter-gen-deprecation')),
+    );
+  });
+
+  testWithoutContext('synthetic-package: true with --no-implicit-pubspec-resolution is an error', () async {
+    // Project directory setup for gen_l10n logic
+    final MemoryFileSystem fileSystem = MemoryFileSystem.test();
+
+    // Add generate:true to pubspec.yaml.
+    final File pubspecFile = fileSystem.file('pubspec.yaml')..createSync();
+    final String content = pubspecFile.readAsStringSync().replaceFirst(
+      '\nflutter:\n',
+      '\nflutter:\n  generate: true\n',
+    );
+    pubspecFile.writeAsStringSync(content);
+
+    // Create an l10n.yaml file
+    fileSystem.file('l10n.yaml').writeAsStringSync('synthetic-package: true');
+
+    final BufferLogger mockBufferLogger = BufferLogger.test();
+    final Environment environment = Environment.test(
+      fileSystem.currentDirectory,
+      fileSystem: fileSystem,
+      logger: mockBufferLogger,
+      artifacts: Artifacts.test(),
+      processManager: FakeProcessManager.any(),
+      useImplicitPubspecResolution: false,
+    );
+    // Will throw if build is called.
+    final TestBuildSystem buildSystem = TestBuildSystem.all(null);
+
+    await expectLater(
+      () => generateLocalizationsSyntheticPackage(
+        environment: environment,
+        buildSystem: buildSystem,
+        buildTargets: const NoOpBuildTargets(),
+      ),
+      throwsToolExit(message: 'Cannot generate a synthetic package when --no-implicit-pubspec-resolution is passed'),
+    );
+  });
+
+  testWithoutContext('synthetic-package defaults to false if --no-implicit-pubspec-resolution is passed', () async {
+    // Project directory setup for gen_l10n logic
+    final MemoryFileSystem fileSystem = MemoryFileSystem.test();
+
+    // Add generate:true to pubspec.yaml.
+    final File pubspecFile = fileSystem.file('pubspec.yaml')..createSync();
+    final String content = pubspecFile.readAsStringSync().replaceFirst(
+      '\nflutter:\n',
+      '\nflutter:\n  generate: true\n',
+    );
+    pubspecFile.writeAsStringSync(content);
+
+    // Create an l10n.yaml file
+    fileSystem.file('l10n.yaml').writeAsStringSync('');
+
+    final BufferLogger mockBufferLogger = BufferLogger.test();
+    final Environment environment = Environment.test(
+      fileSystem.currentDirectory,
+      fileSystem: fileSystem,
+      logger: mockBufferLogger,
+      artifacts: Artifacts.test(),
+      processManager: FakeProcessManager.any(),
+      useImplicitPubspecResolution: false,
+    );
+    // Will throw if build is called.
+    final TestBuildSystem buildSystem = TestBuildSystem.all(null);
+
+    await expectLater(
+      () => generateLocalizationsSyntheticPackage(
+        environment: environment,
+        buildSystem: buildSystem,
+        buildTargets: const NoOpBuildTargets(),
+      ),
+      returnsNormally,
+    );
+  });
 }