diff --git a/docs/plans/2026-03-03-single-instance-enforcement-design.md b/docs/plans/2026-03-03-single-instance-enforcement-design.md
new file mode 100644
index 0000000..f0d819a
--- /dev/null
+++ b/docs/plans/2026-03-03-single-instance-enforcement-design.md
@@ -0,0 +1,70 @@
+# Single-Instance Enforcement Design
+
+## Summary
+
+Prevent multiple instances of the Sendspin Windows client from running simultaneously. When a second instance is launched, it signals the first instance to show its window, then exits silently.
+
+## Motivation
+
+Currently, launching the app when it's already running in the system tray creates a completely separate process with its own audio pipeline, server discovery, tray icon, and clock synchronizer. This confuses users and wastes system resources.
+
+## Design
+
+### Approach: Named Mutex + Named Pipe
+
+**Named Mutex** (`Sendspin_SingleInstance`) detects whether an instance is already running. **Named Pipe** (`Sendspin_ShowWindow`) provides cross-process signaling to bring the existing window to the foreground.
+
+### New File: `src/SendspinClient/SingleInstanceGuard.cs`
+
+An `IDisposable` class with two roles:
+
+**First instance (owns the mutex):**
+1. Creates the named Mutex — `createdNew` is `true`
+2. Starts a `NamedPipeServerStream` listener on a background task
+3. When a connection arrives, raises `ShowWindowRequested` event
+4. Loops: after each connection, creates a new pipe server to listen again
+
+**Second instance (mutex already exists):**
+1. Tries to create the mutex — `createdNew` is `false`
+2. Connects to the named pipe, sends a short message
+3. `TryStart()` returns `false` — caller knows to shut down
+
+### Modified File: `src/SendspinClient/App.xaml.cs`
+
+At the top of `OnStartup`, before DI/logging/window creation:
+
+```csharp
+_singleInstanceGuard = new SingleInstanceGuard();
+if (!_singleInstanceGuard.TryStart())
+{
+    Shutdown();
+    return;
+}
+_singleInstanceGuard.ShowWindowRequested += (_, _) =>
+    Dispatcher.Invoke(() => { /* Show + Activate MainWindow */ });
+```
+
+Guard is disposed in `OnExit` (releases mutex, stops pipe server).
+
+### Window Activation
+
+When `ShowWindowRequested` fires, dispatch to UI thread:
+- `MainWindow.Show()` — unhides from tray
+- `MainWindow.WindowState = WindowState.Normal` — restores if minimized
+- `MainWindow.Activate()` — brings to foreground
+
+### Edge Cases
+
+| Scenario | Behavior |
+|----------|----------|
+| First instance crashes | OS releases kernel mutex; next launch becomes the first instance |
+| First instance exits between mutex check and pipe connect | Pipe connect fails; second instance proceeds as first |
+| Multiple rapid launches | Pipe server loops, handles each connection sequentially |
+| Pipe connect timeout | 2-second timeout; if exceeded, second instance exits without showing (first instance is likely shutting down) |
+
+### No Breaking Changes
+
+- No new dependencies
+- No config changes
+- No UI changes
+- No public API changes
diff --git a/src/SendspinClient/App.xaml.cs b/src/SendspinClient/App.xaml.cs
index 8e51788..a709eef 100644
--- a/src/SendspinClient/App.xaml.cs
+++ b/src/SendspinClient/App.xaml.cs
@@ -27,6 +27,7 @@ namespace SendspinClient;
 /// </summary>
 public partial class App : Application
 {
+    private SingleInstanceGuard? _singleInstanceGuard;
     private ServiceProvider? _serviceProvider;
     private IConfiguration? _configuration;
     private MainViewModel? _mainViewModel;
@@ -47,6 +48,26 @@ protected override void OnStartup(StartupEventArgs e)
     {
         base.OnStartup(e);
 
+        // Single-instance enforcement: if another instance is already running,
+        // signal it to show its window and shut down immediately
+        _singleInstanceGuard = new SingleInstanceGuard();
+        if (!_singleInstanceGuard.TryStart())
+        {
+            Shutdown();
+            return;
+        }
+
+        _singleInstanceGuard.ShowWindowRequested += (_, _) =>
+            Dispatcher.Invoke(() =>
+            {
+                if (MainWindow is { } window)
+                {
+                    window.Show();
+                    window.WindowState = WindowState.Normal;
+                    window.Activate();
+                }
+            });
+
         // Initialize user settings directory (copy defaults on first run)
         AppPaths.InitializeUserSettingsIfNeeded();
 
@@ -560,6 +581,9 @@ protected override async void OnExit(ExitEventArgs e)
     {
         try
         {
+            // Dispose single-instance guard (releases mutex, stops pipe server)
+            _singleInstanceGuard?.Dispose();
+
             // Dispose tray icon to remove from system tray
             _trayIcon?.Dispose();
 
diff --git a/src/SendspinClient/SingleInstanceGuard.cs b/src/SendspinClient/SingleInstanceGuard.cs
new file mode 100644
index 0000000..43083b6
--- /dev/null
+++ b/src/SendspinClient/SingleInstanceGuard.cs
@@ -0,0 +1,113 @@
+using System.IO.Pipes;
+
+namespace SendspinClient;
+
+/// <summary>
+/// Ensures only one instance of the application runs at a time.
+/// Uses a named mutex for detection and a named pipe to signal the
+/// existing instance to show its window.
+/// </summary>
+internal sealed class SingleInstanceGuard : IDisposable
+{
+    private const string MutexName = "Sendspin_SingleInstance";
+    private const string PipeName = "Sendspin_ShowWindow";
+
+    private Mutex? _mutex;
+    private CancellationTokenSource? _pipeCts;
+
+    /// <summary>
+    /// Raised when another instance requests this instance to show its window.
+    /// Always raised on a background thread — callers must dispatch to the UI thread.
+    /// </summary>
+    public event EventHandler? ShowWindowRequested;
+
+    /// <summary>
+    /// Attempts to become the single running instance.
+    /// </summary>
+    /// <returns>
+    /// <c>true</c> if this is the first instance (caller should continue startup).
+    /// <c>false</c> if another instance is already running (caller should shut down).
+    /// </returns>
+    public bool TryStart()
+    {
+        _mutex = new Mutex(initiallyOwned: true, MutexName, out var createdNew);
+
+        if (createdNew)
+        {
+            // We are the first instance — start listening for show-window requests
+            _pipeCts = new CancellationTokenSource();
+            _ = ListenForShowRequestsAsync(_pipeCts.Token);
+            return true;
+        }
+
+        // Another instance owns the mutex — signal it to show its window
+        SignalExistingInstance();
+        return false;
+    }
+
+    /// <summary>
+    /// Listens for incoming pipe connections from subsequent instances.
+    /// Each connection triggers <see cref="ShowWindowRequested"/>.
+    /// </summary>
+    private async Task ListenForShowRequestsAsync(CancellationToken ct)
+    {
+        while (!ct.IsCancellationRequested)
+        {
+            try
+            {
+                await using var server = new NamedPipeServerStream(
+                    PipeName,
+                    PipeDirection.In,
+                    NamedPipeServerStream.MaxAllowedServerInstances,
+                    PipeTransmissionMode.Byte,
+                    PipeOptions.Asynchronous);
+
+                await server.WaitForConnectionAsync(ct);
+
+                // Connection received — raise event (don't need to read data)
+                ShowWindowRequested?.Invoke(this, EventArgs.Empty);
+            }
+            catch (OperationCanceledException)
+            {
+                break;
+            }
+            catch
+            {
+                // Pipe error — wait briefly and retry
+                try { await Task.Delay(500, ct); }
+                catch (OperationCanceledException) { break; }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Connects to the existing instance's named pipe to signal it
+    /// to bring its window to the foreground.
+    /// </summary>
+    private static void SignalExistingInstance()
+    {
+        try
+        {
+            using var client = new NamedPipeClientStream(".", PipeName, PipeDirection.Out);
+            client.Connect(timeout: 2000);
+            // Connection itself is the signal — no data needed
+        }
+        catch
+        {
+            // If pipe connect fails, the existing instance may be shutting down.
+            // Either way, we exit — worst case the user clicks the tray icon.
+        }
+    }
+
+    public void Dispose()
+    {
+        _pipeCts?.Cancel();
+        _pipeCts?.Dispose();
+
+        if (_mutex != null)
+        {
+            try { _mutex.ReleaseMutex(); } catch { }
+            _mutex.Dispose();
+        }
+    }
+}