diff --git a/MainForm.cs b/MainForm.cs
index 3c773e4..dfca223 100644
--- a/MainForm.cs
+++ b/MainForm.cs
@@ -572,6 +572,23 @@ namespace TimerApp
             this.Activate();
         }
 
+        public void ActivateFromExternal()
+        {
+            if (InvokeRequired)
+            {
+                BeginInvoke(new Action(ActivateFromExternal));
+                return;
+            }
+
+            this.Show();
+            this.WindowState = FormWindowState.Normal;
+            this.ShowInTaskbar = true;
+            this.BringToFront();
+            this.Activate();
+            this.TopMost = true;
+            this.TopMost = false;
+        }
+
         private void toolStripMenuItemExit_Click(object sender, EventArgs e)
         {
             _monitor.Stop();
diff --git a/Program.cs b/Program.cs
index 8dc5d8b..67ceea0 100644
--- a/Program.cs
+++ b/Program.cs
@@ -8,11 +8,21 @@ static class Program
     [STAThread]
     static void Main()
     {
-        TaskbarIntegration.InitializeProcess();
-        // To customize application configuration such as set high DPI settings or default font,
-        // see https://aka.ms/applicationconfiguration.
-        ApplicationConfiguration.Initialize();
-        TaskbarIntegration.InitializeShortcuts();
-        Application.Run(new MainForm());
+        if (!SingleInstanceManager.TryAcquire(out var instance) || instance is null)
+        {
+            SingleInstanceManager.SignalExistingInstance();
+            return;
+        }
+
+        using (instance)
+        {
+            TaskbarIntegration.InitializeProcess();
+            ApplicationConfiguration.Initialize();
+            TaskbarIntegration.InitializeShortcuts();
+
+            var mainForm = new MainForm();
+            instance.StartServer(mainForm.ActivateFromExternal);
+            Application.Run(mainForm);
+        }
     }
 }
diff --git a/SingleInstanceManager.cs b/SingleInstanceManager.cs
new file mode 100644
index 0000000..21115d7
--- /dev/null
+++ b/SingleInstanceManager.cs
@@ -0,0 +1,118 @@
+using System;
+using System.IO;
+using System.IO.Pipes;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace TimerApp
+{
+    internal sealed class SingleInstanceManager : IDisposable
+    {
+        private const string MutexName = "Local\\TimerApp.SingleInstance";
+        private const string PipeName = "TimerApp.SingleInstancePipe";
+
+        private readonly Mutex _mutex;
+        private readonly CancellationTokenSource _cts = new();
+
+        private SingleInstanceManager(Mutex mutex)
+        {
+            _mutex = mutex;
+        }
+
+        public static bool TryAcquire(out SingleInstanceManager? manager)
+        {
+            manager = null;
+
+            try
+            {
+                var mutex = new Mutex(true, MutexName, out bool createdNew);
+                if (!createdNew)
+                {
+                    mutex.Dispose();
+                    return false;
+                }
+
+                manager = new SingleInstanceManager(mutex);
+                return true;
+            }
+            catch
+            {
+                return true;
+            }
+        }
+
+        public static void SignalExistingInstance()
+        {
+            try
+            {
+                using var client = new NamedPipeClientStream(".", PipeName, PipeDirection.Out);
+                client.Connect(200);
+                using var writer = new StreamWriter(client) { AutoFlush = true };
+                writer.WriteLine("show");
+            }
+            catch
+            {
+            }
+        }
+
+        public void StartServer(Action onShowRequested)
+        {
+            Task.Run(() => ServerLoop(onShowRequested, _cts.Token));
+        }
+
+        private static async Task ServerLoop(Action onShowRequested, CancellationToken cancellationToken)
+        {
+            while (!cancellationToken.IsCancellationRequested)
+            {
+                try
+                {
+                    using var server = new NamedPipeServerStream(
+                        PipeName,
+                        PipeDirection.In,
+                        1,
+                        PipeTransmissionMode.Byte,
+                        PipeOptions.Asynchronous
+                    );
+
+                    await server.WaitForConnectionAsync(cancellationToken).ConfigureAwait(false);
+
+                    using var reader = new StreamReader(server);
+                    string? line = await reader.ReadLineAsync(cancellationToken).ConfigureAwait(false);
+                    if (string.Equals(line, "show", StringComparison.OrdinalIgnoreCase))
+                        onShowRequested();
+                }
+                catch (OperationCanceledException)
+                {
+                    return;
+                }
+                catch
+                {
+                    await Task.Delay(150, cancellationToken).ConfigureAwait(false);
+                }
+            }
+        }
+
+        public void Dispose()
+        {
+            try
+            {
+                _cts.Cancel();
+            }
+            catch
+            {
+            }
+
+            try
+            {
+                _mutex.ReleaseMutex();
+            }
+            catch
+            {
+            }
+
+            _mutex.Dispose();
+            _cts.Dispose();
+        }
+    }
+}
+