diff --git a/src/NATS.Server/Monitoring/ClosedClient.cs b/src/NATS.Server/Monitoring/ClosedClient.cs
index 0710d19..277fba8 100644
--- a/src/NATS.Server/Monitoring/ClosedClient.cs
+++ b/src/NATS.Server/Monitoring/ClosedClient.cs
@@ -22,4 +22,5 @@ public sealed record ClosedClient
     public TimeSpan Rtt { get; init; }
     public string TlsVersion { get; init; } = "";
     public string TlsCipherSuite { get; init; } = "";
+    public string MqttClient { get; init; } = "";
 }
diff --git a/src/NATS.Server/Monitoring/Connz.cs b/src/NATS.Server/Monitoring/Connz.cs
index aae62ed..cea93ca 100644
--- a/src/NATS.Server/Monitoring/Connz.cs
+++ b/src/NATS.Server/Monitoring/Connz.cs
@@ -204,6 +204,8 @@ public sealed class ConnzOptions
 
     public string FilterSubject { get; set; } = "";
 
+    public string MqttClient { get; set; } = "";
+
     public int Offset { get; set; }
 
     public int Limit { get; set; } = 1024;
diff --git a/src/NATS.Server/Monitoring/ConnzHandler.cs b/src/NATS.Server/Monitoring/ConnzHandler.cs
index 8ecf512..96c96de 100644
--- a/src/NATS.Server/Monitoring/ConnzHandler.cs
+++ b/src/NATS.Server/Monitoring/ConnzHandler.cs
@@ -28,6 +28,10 @@ public sealed class ConnzHandler(NatsServer server)
             connInfos.AddRange(server.GetClosedClients().Select(c => BuildClosedConnInfo(c, now, opts)));
         }
 
+        // Filter by MQTT client ID
+        if (!string.IsNullOrEmpty(opts.MqttClient))
+            connInfos = connInfos.Where(c => c.MqttClient == opts.MqttClient).ToList();
+
         // Validate sort options that require closed state
         if (opts.Sort is SortOpt.ByStop or SortOpt.ByReason && opts.State == ConnState.Open)
             opts.Sort = SortOpt.ByCid; // Fallback
@@ -142,6 +146,7 @@ public sealed class ConnzHandler(NatsServer server)
             Rtt = FormatRtt(closed.Rtt),
             TlsVersion = closed.TlsVersion,
             TlsCipherSuite = closed.TlsCipherSuite,
+            MqttClient = closed.MqttClient,
         };
     }
 
@@ -197,6 +202,9 @@ public sealed class ConnzHandler(NatsServer server)
         if (q.TryGetValue("limit", out var limit) && int.TryParse(limit, out var l))
             opts.Limit = l;
 
+        if (q.TryGetValue("mqtt_client", out var mqttClient))
+            opts.MqttClient = mqttClient.ToString();
+
         return opts;
     }
 
diff --git a/src/NATS.Server/NatsServer.cs b/src/NATS.Server/NatsServer.cs
index 9a1f717..f49a6c4 100644
--- a/src/NATS.Server/NatsServer.cs
+++ b/src/NATS.Server/NatsServer.cs
@@ -837,6 +837,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             Rtt = client.Rtt,
             TlsVersion = client.TlsState?.TlsVersion ?? "",
             TlsCipherSuite = client.TlsState?.CipherSuite ?? "",
+            MqttClient = "", // populated when MQTT transport is implemented
         });
 
         // Cap closed clients list
diff --git a/tests/NATS.Server.Tests/MonitorTests.cs b/tests/NATS.Server.Tests/MonitorTests.cs
index e89a0db..d4dad6c 100644
--- a/tests/NATS.Server.Tests/MonitorTests.cs
+++ b/tests/NATS.Server.Tests/MonitorTests.cs
@@ -203,6 +203,51 @@ public class MonitorTests : IAsyncLifetime
         closed.Reason.ShouldNotBeNullOrEmpty();
     }
 
+    [Fact]
+    public async Task Connz_filters_by_mqtt_client_for_open_connections()
+    {
+        // Connect a regular NATS client (no MQTT ID)
+        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await sock.ConnectAsync(new IPEndPoint(IPAddress.Loopback, _natsPort));
+        using var stream = new NetworkStream(sock);
+        var buf = new byte[4096];
+        _ = await stream.ReadAsync(buf);
+        await stream.WriteAsync("CONNECT {}\r\n"u8.ToArray());
+        await Task.Delay(200);
+
+        // Query for an MQTT client ID that no connection has
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/connz?mqtt_client=some-id");
+        response.StatusCode.ShouldBe(HttpStatusCode.OK);
+
+        var connz = await response.Content.ReadFromJsonAsync<Connz>();
+        connz.ShouldNotBeNull();
+        connz.NumConns.ShouldBe(0);
+    }
+
+    [Fact]
+    public async Task Connz_filters_by_mqtt_client_for_closed_connections()
+    {
+        // Connect then disconnect a client so it appears in closed list
+        var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await sock.ConnectAsync(new IPEndPoint(IPAddress.Loopback, _natsPort));
+        using var stream = new NetworkStream(sock);
+        var buf = new byte[4096];
+        _ = await stream.ReadAsync(buf);
+        await stream.WriteAsync("CONNECT {}\r\n"u8.ToArray());
+        await Task.Delay(200);
+        sock.Shutdown(SocketShutdown.Both);
+        sock.Dispose();
+        await Task.Delay(500);
+
+        // Query closed connections with an MQTT client ID that no connection has
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/connz?state=closed&mqtt_client=missing-id");
+        response.StatusCode.ShouldBe(HttpStatusCode.OK);
+
+        var connz = await response.Content.ReadFromJsonAsync<Connz>();
+        connz.ShouldNotBeNull();
+        connz.NumConns.ShouldBe(0);
+    }
+
     [Fact]
     public async Task Connz_sort_by_stop_requires_closed_state()
     {