diff --git a/docs/SETUP.md b/docs/SETUP.md
index 5015df2..db7b850 100644
--- a/docs/SETUP.md
+++ b/docs/SETUP.md
@@ -30,6 +30,9 @@ enabled by default.
 In REST mode, auto-triggers are reachability-gated to avoid unnecessary sync
 attempts while offline.
 
+When connectivity returns (offline -> online), sync now triggers immediately in
+addition to startup/resume/periodic ticks.
+
 Control flags:
 
 ```bash
diff --git a/docs/progress.md b/docs/progress.md
index f51f5ea..779a4f4 100644
--- a/docs/progress.md
+++ b/docs/progress.md
@@ -2,6 +2,34 @@
 
 Updated: 2026-02-17
 
+## Latest Milestone (2026-02-17): Connectivity Event-Driven Background Sync
+
+Implemented online-transition-triggered sync so background sync reacts faster
+when network returns:
+
+- Added connectivity stream support to reachability abstraction:
+  - `lib/core/network/reachability/network_reachability.dart`
+  - new `watch(...)` API for reachability change events
+- Added IO implementation using connectivity plugin + internet probe:
+  - `lib/core/network/reachability/network_reachability_io.dart`
+  - `connectivity_plus` integrated as transport signal source
+- Updated background runner:
+  - `lib/features/sync/sync_background_runner.dart`
+  - now listens for offline -> online transitions and triggers immediate sync
+  - keeps existing startup/resume/periodic behavior
+- Added coverage:
+  - `test/features/sync/sync_background_runner_test.dart`
+  - verifies transition-trigger behavior and avoids duplicate triggers when
+    already online
+- Updated setup docs:
+  - `docs/SETUP.md` background sync section now includes connectivity-return
+    behavior note
+
+Validation for this milestone:
+
+- `flutter analyze` -> pass
+- `flutter test` -> pass
+
 ## Latest Milestone (2026-02-17): Sync Repair Test Stability + Mobile CRUD/Sync Flow Coverage
 
 Completed two test-depth upgrades:
@@ -313,8 +341,8 @@ Validation for this milestone:
    - future enhancement: auto-select and highlight the exact row in main feature
      screens after save.
 3. Sync trigger maturity:
-   - implemented via reachability gating; future improvement is explicit OS
-     connectivity event subscription.
+   - implemented via reachability gating + online-transition event triggers.
+   - future: add richer platform-specific network quality awareness if needed.
 4. Test depth:
    - expanded with mobile compact CRUD+sync flow and rejected-repair flow.
    - future: add desktop + web-specific end-to-end sync error/retry scenario.
diff --git a/lib/core/network/reachability/network_reachability.dart b/lib/core/network/reachability/network_reachability.dart
index 39a5401..aa90655 100644
--- a/lib/core/network/reachability/network_reachability.dart
+++ b/lib/core/network/reachability/network_reachability.dart
@@ -6,6 +6,9 @@ import 'package:relationship_saver/core/network/reachability/network_reachabilit
 abstract class NetworkReachability {
   /// Returns `true` when network looks reachable.
   Future<bool> isReachable({Duration timeout = const Duration(seconds: 2)});
+
+  /// Emits reachability updates when platform network status changes.
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)});
 }
 
 /// Returns platform implementation for network reachability probing.
diff --git a/lib/core/network/reachability/network_reachability_io.dart b/lib/core/network/reachability/network_reachability_io.dart
index e616181..063a2ed 100644
--- a/lib/core/network/reachability/network_reachability_io.dart
+++ b/lib/core/network/reachability/network_reachability_io.dart
@@ -1,9 +1,15 @@
 import 'dart:async';
 import 'dart:io';
 
+import 'package:connectivity_plus/connectivity_plus.dart';
 import 'package:relationship_saver/core/network/reachability/network_reachability.dart';
 
 class IoNetworkReachability implements NetworkReachability {
+  IoNetworkReachability({Connectivity? connectivity})
+    : _connectivity = connectivity ?? Connectivity();
+
+  final Connectivity _connectivity;
+
   @override
   Future<bool> isReachable({
     Duration timeout = const Duration(seconds: 2),
@@ -19,6 +25,35 @@ class IoNetworkReachability implements NetworkReachability {
       return false;
     }
   }
+
+  @override
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)}) {
+    final Stream<dynamic> changes =
+        _connectivity.onConnectivityChanged as Stream<dynamic>;
+    return changes
+        .asyncMap((dynamic event) async {
+          final Iterable<ConnectivityResult> results =
+              _normalizeConnectivityResults(event);
+          final bool hasTransport = results.any(
+            (ConnectivityResult result) => result != ConnectivityResult.none,
+          );
+          if (!hasTransport) {
+            return false;
+          }
+          return isReachable(timeout: timeout);
+        })
+        .distinct();
+  }
+
+  Iterable<ConnectivityResult> _normalizeConnectivityResults(dynamic event) {
+    if (event is ConnectivityResult) {
+      return <ConnectivityResult>[event];
+    }
+    if (event is List<ConnectivityResult>) {
+      return event;
+    }
+    return const <ConnectivityResult>[];
+  }
 }
 
 NetworkReachability createPlatformNetworkReachability() =>
diff --git a/lib/core/network/reachability/network_reachability_stub.dart b/lib/core/network/reachability/network_reachability_stub.dart
index 8926156..9bfac69 100644
--- a/lib/core/network/reachability/network_reachability_stub.dart
+++ b/lib/core/network/reachability/network_reachability_stub.dart
@@ -7,6 +7,11 @@ class StubNetworkReachability implements NetworkReachability {
   }) async {
     return true;
   }
+
+  @override
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)}) {
+    return const Stream<bool>.empty();
+  }
 }
 
 NetworkReachability createPlatformNetworkReachability() =>
diff --git a/lib/core/network/reachability/network_reachability_web.dart b/lib/core/network/reachability/network_reachability_web.dart
index 4842d88..a4e66be 100644
--- a/lib/core/network/reachability/network_reachability_web.dart
+++ b/lib/core/network/reachability/network_reachability_web.dart
@@ -8,6 +8,11 @@ class WebNetworkReachability implements NetworkReachability {
     // Web environments vary in available probes; keep optimistic fallback.
     return true;
   }
+
+  @override
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)}) {
+    return const Stream<bool>.empty();
+  }
 }
 
 NetworkReachability createPlatformNetworkReachability() =>
diff --git a/lib/features/sync/sync_background_runner.dart b/lib/features/sync/sync_background_runner.dart
index 9686dea..6eae24d 100644
--- a/lib/features/sync/sync_background_runner.dart
+++ b/lib/features/sync/sync_background_runner.dart
@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:relationship_saver/core/config/app_config.dart';
+import 'package:relationship_saver/core/network/reachability/network_reachability.dart';
 import 'package:relationship_saver/core/network/reachability/network_reachability_provider.dart';
 import 'package:relationship_saver/features/sync/sync_auto_trigger_controller.dart';
 import 'package:relationship_saver/features/sync/sync_coordinator.dart';
@@ -22,10 +23,13 @@ class _SyncBackgroundRunnerState extends ConsumerState<SyncBackgroundRunner>
     with WidgetsBindingObserver {
   late final SyncAutoTriggerController _controller;
   Timer? _periodicTimer;
+  StreamSubscription<bool>? _reachabilitySubscription;
+  bool? _lastReachable;
 
   @override
   void initState() {
     super.initState();
+    final reachability = ref.read(networkReachabilityProvider);
     _controller = SyncAutoTriggerController(
       syncNow: () => ref.read(syncCoordinatorProvider).syncNow(),
       now: DateTime.now,
@@ -36,11 +40,12 @@ class _SyncBackgroundRunnerState extends ConsumerState<SyncBackgroundRunner>
         if (AppConfig.useFakeBackend) {
           return true;
         }
-        return ref.read(networkReachabilityProvider).isReachable();
+        return reachability.isReachable();
       },
     );
     WidgetsBinding.instance.addObserver(this);
     _configurePeriodicRunner();
+    _configureReachabilityRunner(reachability);
     if (AppConfig.enableBackgroundSync) {
       unawaited(_controller.trigger(ignoreCooldown: true));
     }
@@ -60,6 +65,7 @@ class _SyncBackgroundRunnerState extends ConsumerState<SyncBackgroundRunner>
   void dispose() {
     WidgetsBinding.instance.removeObserver(this);
     _periodicTimer?.cancel();
+    _reachabilitySubscription?.cancel();
     super.dispose();
   }
 
@@ -80,4 +86,22 @@ class _SyncBackgroundRunnerState extends ConsumerState<SyncBackgroundRunner>
       unawaited(_controller.trigger());
     });
   }
+
+  void _configureReachabilityRunner(NetworkReachability reachability) {
+    if (!AppConfig.enableBackgroundSync) {
+      return;
+    }
+    _reachabilitySubscription = reachability.watch().listen((bool reachable) {
+      final bool? previous = _lastReachable;
+      _lastReachable = reachable;
+      if (reachable && previous == false) {
+        unawaited(_controller.trigger(ignoreCooldown: true));
+      }
+    });
+    unawaited(_primeReachabilityState(reachability));
+  }
+
+  Future<void> _primeReachabilityState(NetworkReachability reachability) async {
+    _lastReachable = await reachability.isReachable();
+  }
 }
diff --git a/macos/Flutter/GeneratedPluginRegistrant.swift b/macos/Flutter/GeneratedPluginRegistrant.swift
index 157bb5d..4ce058e 100644
--- a/macos/Flutter/GeneratedPluginRegistrant.swift
+++ b/macos/Flutter/GeneratedPluginRegistrant.swift
@@ -5,11 +5,13 @@
 import FlutterMacOS
 import Foundation
 
+import connectivity_plus
 import flutter_local_notifications
 import flutter_secure_storage_darwin
 import shared_preferences_foundation
 
 func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
+  ConnectivityPlusPlugin.register(with: registry.registrar(forPlugin: "ConnectivityPlusPlugin"))
   FlutterLocalNotificationsPlugin.register(with: registry.registrar(forPlugin: "FlutterLocalNotificationsPlugin"))
   FlutterSecureStorageDarwinPlugin.register(with: registry.registrar(forPlugin: "FlutterSecureStorageDarwinPlugin"))
   SharedPreferencesPlugin.register(with: registry.registrar(forPlugin: "SharedPreferencesPlugin"))
diff --git a/pubspec.lock b/pubspec.lock
index d7e7dc6..71ed454 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -145,6 +145,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.19.1"
+  connectivity_plus:
+    dependency: "direct main"
+    description:
+      name: connectivity_plus
+      sha256: "33bae12a398f841c6cda09d1064212957265869104c478e5ad51e2fb26c3973c"
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.0.0"
+  connectivity_plus_platform_interface:
+    dependency: transitive
+    description:
+      name: connectivity_plus_platform_interface
+      sha256: "42657c1715d48b167930d5f34d00222ac100475f73d10162ddf43e714932f204"
+      url: "https://pub.dev"
+    source: hosted
+    version: "2.0.1"
   convert:
     dependency: transitive
     description:
@@ -560,6 +576,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.17.4"
+  nm:
+    dependency: transitive
+    description:
+      name: nm
+      sha256: "2c9aae4127bdc8993206464fcc063611e0e36e72018696cd9631023a31b24254"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.5.0"
   node_preamble:
     dependency: transitive
     description:
diff --git a/pubspec.yaml b/pubspec.yaml
index ecce72e..548dd88 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -43,6 +43,7 @@ dependencies:
   flutter_riverpod: ^3.2.1
   shared_preferences: ^2.5.4
   hive_flutter: ^1.1.0
+  connectivity_plus: ^7.0.0
   timezone: ^0.10.1
   flutter_local_notifications: ^20.1.0
 
diff --git a/test/features/sync/sync_background_runner_test.dart b/test/features/sync/sync_background_runner_test.dart
new file mode 100644
index 0000000..65e7ae2
--- /dev/null
+++ b/test/features/sync/sync_background_runner_test.dart
@@ -0,0 +1,131 @@
+import 'dart:async';
+
+import 'package:flutter/widgets.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:relationship_saver/core/network/reachability/network_reachability.dart';
+import 'package:relationship_saver/core/network/reachability/network_reachability_provider.dart';
+import 'package:relationship_saver/features/sync/sync_background_runner.dart';
+import 'package:relationship_saver/features/sync/sync_coordinator.dart';
+
+void main() {
+  testWidgets('triggers sync when reachability transitions from offline to online', (
+    WidgetTester tester,
+  ) async {
+    final _TestReachability reachability = _TestReachability(initial: false);
+    addTearDown(reachability.dispose);
+
+    int syncCalls = 0;
+
+    await tester.pumpWidget(
+      ProviderScope(
+        overrides: [
+          networkReachabilityProvider.overrideWithValue(reachability),
+          syncCoordinatorProvider.overrideWith(
+            (Ref ref) =>
+                _TestSyncCoordinator(ref, onSyncNow: () async {
+                  syncCalls += 1;
+                  return _okResult();
+                }),
+          ),
+        ],
+        child: const Directionality(
+          textDirection: TextDirection.ltr,
+          child: SyncBackgroundRunner(child: SizedBox()),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle(const Duration(milliseconds: 60));
+
+    // Startup trigger always runs once when background sync is enabled.
+    expect(syncCalls, 1);
+
+    reachability.emit(false);
+    await tester.pumpAndSettle(const Duration(milliseconds: 40));
+    expect(syncCalls, 1);
+
+    reachability.emit(true);
+    await tester.pumpAndSettle(const Duration(milliseconds: 60));
+    expect(syncCalls, 2);
+  });
+
+  testWidgets('does not trigger extra sync while connectivity stays online', (
+    WidgetTester tester,
+  ) async {
+    final _TestReachability reachability = _TestReachability(initial: true);
+    addTearDown(reachability.dispose);
+
+    int syncCalls = 0;
+
+    await tester.pumpWidget(
+      ProviderScope(
+        overrides: [
+          networkReachabilityProvider.overrideWithValue(reachability),
+          syncCoordinatorProvider.overrideWith(
+            (Ref ref) =>
+                _TestSyncCoordinator(ref, onSyncNow: () async {
+                  syncCalls += 1;
+                  return _okResult();
+                }),
+          ),
+        ],
+        child: const Directionality(
+          textDirection: TextDirection.ltr,
+          child: SyncBackgroundRunner(child: SizedBox()),
+        ),
+      ),
+    );
+    await tester.pumpAndSettle(const Duration(milliseconds: 60));
+
+    expect(syncCalls, 1);
+
+    reachability.emit(true);
+    await tester.pumpAndSettle(const Duration(milliseconds: 60));
+
+    expect(syncCalls, 1);
+  });
+}
+
+class _TestSyncCoordinator extends SyncCoordinator {
+  _TestSyncCoordinator(super.ref, {required this.onSyncNow});
+
+  final Future<SyncRunResult> Function() onSyncNow;
+
+  @override
+  Future<SyncRunResult> syncNow() => onSyncNow();
+}
+
+class _TestReachability implements NetworkReachability {
+  _TestReachability({required bool initial}) : _current = initial;
+
+  final StreamController<bool> _controller = StreamController<bool>.broadcast();
+  bool _current;
+
+  @override
+  Future<bool> isReachable({Duration timeout = const Duration(seconds: 2)}) async {
+    return _current;
+  }
+
+  @override
+  Stream<bool> watch({Duration timeout = const Duration(seconds: 2)}) {
+    return _controller.stream;
+  }
+
+  void emit(bool reachable) {
+    _current = reachable;
+    _controller.add(reachable);
+  }
+
+  Future<void> dispose() async {
+    await _controller.close();
+  }
+}
+
+SyncRunResult _okResult() {
+  return const SyncRunResult(
+    success: true,
+    operation: 'sync',
+    message: 'ok',
+    pendingAfter: 0,
+  );
+}
diff --git a/windows/flutter/generated_plugin_registrant.cc b/windows/flutter/generated_plugin_registrant.cc
index 0c50753..af1f996 100644
--- a/windows/flutter/generated_plugin_registrant.cc
+++ b/windows/flutter/generated_plugin_registrant.cc
@@ -6,9 +6,12 @@
 
 #include "generated_plugin_registrant.h"
 
+#include <connectivity_plus/connectivity_plus_windows_plugin.h>
 #include <flutter_secure_storage_windows/flutter_secure_storage_windows_plugin.h>
 
 void RegisterPlugins(flutter::PluginRegistry* registry) {
+  ConnectivityPlusWindowsPluginRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("ConnectivityPlusWindowsPlugin"));
   FlutterSecureStorageWindowsPluginRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("FlutterSecureStorageWindowsPlugin"));
 }
diff --git a/windows/flutter/generated_plugins.cmake b/windows/flutter/generated_plugins.cmake
index 197d43d..71d65ea 100644
--- a/windows/flutter/generated_plugins.cmake
+++ b/windows/flutter/generated_plugins.cmake
@@ -3,6 +3,7 @@
 #
 
 list(APPEND FLUTTER_PLUGIN_LIST
+  connectivity_plus
   flutter_secure_storage_windows
 )