diff --git a/IMPLEMENTATION.md b/IMPLEMENTATION.md
index f588a6c..3619bf9 100644
--- a/IMPLEMENTATION.md
+++ b/IMPLEMENTATION.md
@@ -167,7 +167,7 @@ Each type lives in `src/FrameProcessor/Domain/`. Tests in `tests/FrameProcessor.
 ### [x] 6.4 `/health` reports MQTT status
 - Replace hardcoded `mqttConnected` with `MqttPublisher.IsConnected`.
 
-### [ ] 6.5 Background retry queue
+### [x] 6.5 Background retry queue
 - In-memory `Channel<MacAddress>` (one slot per frame; newer publish supersedes older — per `SPEC.md` §5.1 "Multiple queued publishes for the same frame collapse to the most recent one").
 - Background loop drains with backoff sequence from `MqttOptions.RetryBackoffSeconds`.
 - On reconnect, drain immediately.
diff --git a/src/FrameProcessor/Mqtt/MqttPublisher.cs b/src/FrameProcessor/Mqtt/MqttPublisher.cs
index 95e5f63..83d20ca 100644
--- a/src/FrameProcessor/Mqtt/MqttPublisher.cs
+++ b/src/FrameProcessor/Mqtt/MqttPublisher.cs
@@ -1,3 +1,5 @@
+using System.Collections.Concurrent;
+using System.Threading.Channels;
 using FrameProcessor.Configuration;
 using FrameProcessor.Domain;
 using Microsoft.Extensions.Options;
@@ -11,6 +13,11 @@ namespace FrameProcessor.Mqtt;
 /// Persistent MQTT client wrapped in a hosted service. Connects on startup and
 /// reconnects in the background after disconnects. Connection state is surfaced
 /// via <see cref="IsConnected"/> for the <c>/health</c> endpoint.
+///
+/// On publish failure the (mac → topic, payload) pair is enqueued for retry; a
+/// background loop drains the queue using <see cref="MqttOptions.RetryBackoffSeconds"/>.
+/// Multiple failed publishes for the same MAC collapse to a single pending entry
+/// per SPEC.md §5.1.
 /// </summary>
 public sealed class MqttPublisher : BackgroundService
 {
@@ -20,6 +27,10 @@ public sealed class MqttPublisher : BackgroundService
     private readonly ILogger<MqttPublisher> _logger;
     private readonly IMqttClient _client;
 
+    private readonly ConcurrentDictionary<MacAddress, byte> _pending = new();
+    private readonly Channel<bool> _wakeup = Channel.CreateBounded<bool>(
+        new BoundedChannelOptions(1) { FullMode = BoundedChannelFullMode.DropWrite });
+
     public MqttPublisher(IOptions<MqttOptions> options, ILogger<MqttPublisher> logger)
     {
         _options = options;
@@ -53,39 +64,50 @@ public sealed class MqttPublisher : BackgroundService
 
     protected override async Task ExecuteAsync(CancellationToken stoppingToken)
     {
-        while (!stoppingToken.IsCancellationRequested)
+        var retryLoop = RunRetryLoopAsync(stoppingToken);
+
+        try
         {
-            try
+            while (!stoppingToken.IsCancellationRequested)
             {
-                await Task.Delay(ReconnectDelay, stoppingToken).ConfigureAwait(false);
-            }
-            catch (OperationCanceledException)
-            {
-                return;
-            }
+                try
+                {
+                    await Task.Delay(ReconnectDelay, stoppingToken).ConfigureAwait(false);
+                }
+                catch (OperationCanceledException)
+                {
+                    return;
+                }
 
-            if (_client.IsConnected)
-            {
-                continue;
-            }
+                if (_client.IsConnected)
+                {
+                    continue;
+                }
 
-            try
-            {
-                await _client.ConnectAsync(BuildClientOptions(), stoppingToken).ConfigureAwait(false);
-                _logger.LogInformation(
-                    "MQTT reconnected to {Host}:{Port}",
-                    _options.Value.Host,
-                    _options.Value.Port);
-            }
-            catch (Exception ex) when (!stoppingToken.IsCancellationRequested)
-            {
-                _logger.LogWarning(
-                    ex,
-                    "MQTT reconnect to {Host}:{Port} failed",
-                    _options.Value.Host,
-                    _options.Value.Port);
+                try
+                {
+                    await _client.ConnectAsync(BuildClientOptions(), stoppingToken).ConfigureAwait(false);
+                    _logger.LogInformation(
+                        "MQTT reconnected to {Host}:{Port}",
+                        _options.Value.Host,
+                        _options.Value.Port);
+                    // Drain any queued publishes immediately now that the link is back up.
+                    _wakeup.Writer.TryWrite(true);
+                }
+                catch (Exception ex) when (!stoppingToken.IsCancellationRequested)
+                {
+                    _logger.LogWarning(
+                        ex,
+                        "MQTT reconnect to {Host}:{Port} failed",
+                        _options.Value.Host,
+                        _options.Value.Port);
+                }
             }
         }
+        finally
+        {
+            await retryLoop.ConfigureAwait(false);
+        }
     }
 
     /// <summary>
@@ -94,11 +116,30 @@ public sealed class MqttPublisher : BackgroundService
     /// transport fault) — per SPEC.md §3.5, publish failure must not fail the upload,
     /// so this method never throws on MQTT errors. <see cref="OperationCanceledException"/>
     /// still propagates so callers can honor cooperative cancellation.
+    ///
+    /// On failure, the MAC is enqueued for background retry (SPEC.md §5.1).
     /// </summary>
     public async Task<PublishResult> PublishAsync(MacAddress mac, CancellationToken cancellationToken)
+    {
+        var result = await PublishCoreAsync(mac, cancellationToken).ConfigureAwait(false);
+        if (result == PublishResult.Failure)
+        {
+            EnqueueRetry(mac);
+        }
+        return result;
+    }
+
+    private async Task<PublishResult> PublishCoreAsync(MacAddress mac, CancellationToken cancellationToken)
     {
         var opts = _options.Value;
         var topic = $"{opts.BaseTopic}/{mac}";
+
+        if (!_client.IsConnected)
+        {
+            _logger.LogWarning("MQTT publish to {Topic} skipped — client not connected", topic);
+            return PublishResult.Failure;
+        }
+
         var message = new MqttApplicationMessageBuilder()
             .WithTopic(topic)
             .WithPayload("update")
@@ -125,6 +166,94 @@ public sealed class MqttPublisher : BackgroundService
         }
     }
 
+    private void EnqueueRetry(MacAddress mac)
+    {
+        _pending[mac] = 0;
+        _wakeup.Writer.TryWrite(true);
+    }
+
+    private async Task RunRetryLoopAsync(CancellationToken stoppingToken)
+    {
+        var backoff = _options.Value.RetryBackoffSeconds;
+        var backoffIndex = 0;
+
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            try
+            {
+                if (_pending.IsEmpty)
+                {
+                    // Idle: wait indefinitely for an enqueue or a reconnect signal.
+                    await _wakeup.Reader.ReadAsync(stoppingToken).ConfigureAwait(false);
+                    backoffIndex = 0;
+                }
+                else
+                {
+                    var delaySeconds = backoff[Math.Min(backoffIndex, backoff.Length - 1)];
+                    using var delayCts = CancellationTokenSource.CreateLinkedTokenSource(stoppingToken);
+                    delayCts.CancelAfter(TimeSpan.FromSeconds(delaySeconds));
+                    try
+                    {
+                        // A signal during the backoff (new failure, reconnect) wakes us early.
+                        await _wakeup.Reader.ReadAsync(delayCts.Token).ConfigureAwait(false);
+                    }
+                    catch (OperationCanceledException) when (!stoppingToken.IsCancellationRequested)
+                    {
+                        // Backoff window elapsed; fall through to attempt.
+                    }
+                }
+            }
+            catch (OperationCanceledException)
+            {
+                return;
+            }
+
+            if (stoppingToken.IsCancellationRequested)
+            {
+                return;
+            }
+
+            var anyAttempted = false;
+            var anyFailed = false;
+            foreach (var mac in _pending.Keys.ToArray())
+            {
+                if (!_client.IsConnected)
+                {
+                    // Wait for the reconnect loop to bring the link back up before retrying.
+                    anyFailed = true;
+                    break;
+                }
+
+                anyAttempted = true;
+                var result = await PublishCoreAsync(mac, stoppingToken).ConfigureAwait(false);
+                if (result == PublishResult.Success)
+                {
+                    _pending.TryRemove(mac, out _);
+                }
+                else
+                {
+                    anyFailed = true;
+                }
+            }
+
+            if (!anyAttempted || anyFailed)
+            {
+                if (!_pending.IsEmpty)
+                {
+                    backoffIndex = Math.Min(backoffIndex + 1, backoff.Length - 1);
+                }
+                else
+                {
+                    backoffIndex = 0;
+                }
+            }
+            else
+            {
+                backoffIndex = 0;
+            }
+        }
+    }
+
     public override async Task StopAsync(CancellationToken cancellationToken)
     {
         await base.StopAsync(cancellationToken).ConfigureAwait(false);