diff --git a/src/NATS.Server/Monitoring/ConnzHandler.cs b/src/NATS.Server/Monitoring/ConnzHandler.cs
new file mode 100644
index 0000000..e5cd7b0
--- /dev/null
+++ b/src/NATS.Server/Monitoring/ConnzHandler.cs
@@ -0,0 +1,148 @@
+using Microsoft.AspNetCore.Http;
+
+namespace NATS.Server.Monitoring;
+
+/// <summary>
+/// Handles /connz endpoint requests, returning detailed connection information.
+/// Corresponds to Go server/monitor.go handleConnz function.
+/// </summary>
+public sealed class ConnzHandler(NatsServer server)
+{
+    public Connz HandleConnz(HttpContext ctx)
+    {
+        var opts = ParseQueryParams(ctx);
+        var now = DateTime.UtcNow;
+        var clients = server.GetClients().ToArray();
+
+        var connInfos = clients.Select(c => BuildConnInfo(c, now, opts)).ToList();
+
+        // Sort
+        connInfos = opts.Sort switch
+        {
+            SortOpt.ByCid => connInfos.OrderBy(c => c.Cid).ToList(),
+            SortOpt.ByStart => connInfos.OrderBy(c => c.Start).ToList(),
+            SortOpt.BySubs => connInfos.OrderByDescending(c => c.NumSubs).ToList(),
+            SortOpt.ByPending => connInfos.OrderByDescending(c => c.Pending).ToList(),
+            SortOpt.ByMsgsTo => connInfos.OrderByDescending(c => c.OutMsgs).ToList(),
+            SortOpt.ByMsgsFrom => connInfos.OrderByDescending(c => c.InMsgs).ToList(),
+            SortOpt.ByBytesTo => connInfos.OrderByDescending(c => c.OutBytes).ToList(),
+            SortOpt.ByBytesFrom => connInfos.OrderByDescending(c => c.InBytes).ToList(),
+            SortOpt.ByLast => connInfos.OrderByDescending(c => c.LastActivity).ToList(),
+            SortOpt.ByIdle => connInfos.OrderByDescending(c => now - c.LastActivity).ToList(),
+            SortOpt.ByUptime => connInfos.OrderByDescending(c => now - c.Start).ToList(),
+            _ => connInfos.OrderBy(c => c.Cid).ToList(),
+        };
+
+        var total = connInfos.Count;
+        var paged = connInfos.Skip(opts.Offset).Take(opts.Limit).ToArray();
+
+        return new Connz
+        {
+            Id = server.ServerId,
+            Now = now,
+            NumConns = paged.Length,
+            Total = total,
+            Offset = opts.Offset,
+            Limit = opts.Limit,
+            Conns = paged,
+        };
+    }
+
+    private static ConnInfo BuildConnInfo(NatsClient client, DateTime now, ConnzOptions opts)
+    {
+        var info = new ConnInfo
+        {
+            Cid = client.Id,
+            Kind = "Client",
+            Type = "Client",
+            Ip = client.RemoteIp ?? "",
+            Port = client.RemotePort,
+            Start = client.StartTime,
+            LastActivity = client.LastActivity,
+            Uptime = FormatDuration(now - client.StartTime),
+            Idle = FormatDuration(now - client.LastActivity),
+            InMsgs = Interlocked.Read(ref client.InMsgs),
+            OutMsgs = Interlocked.Read(ref client.OutMsgs),
+            InBytes = Interlocked.Read(ref client.InBytes),
+            OutBytes = Interlocked.Read(ref client.OutBytes),
+            NumSubs = (uint)client.Subscriptions.Count,
+            Name = client.ClientOpts?.Name ?? "",
+            Lang = client.ClientOpts?.Lang ?? "",
+            Version = client.ClientOpts?.Version ?? "",
+            TlsVersion = client.TlsState?.TlsVersion ?? "",
+            TlsCipherSuite = client.TlsState?.CipherSuite ?? "",
+        };
+
+        if (opts.Subscriptions)
+        {
+            info.Subs = client.Subscriptions.Values.Select(s => s.Subject).ToArray();
+        }
+
+        if (opts.SubscriptionsDetail)
+        {
+            info.SubsDetail = client.Subscriptions.Values.Select(s => new SubDetail
+            {
+                Subject = s.Subject,
+                Queue = s.Queue ?? "",
+                Sid = s.Sid,
+                Msgs = Interlocked.Read(ref s.MessageCount),
+                Max = s.MaxMessages,
+                Cid = client.Id,
+            }).ToArray();
+        }
+
+        return info;
+    }
+
+    private static ConnzOptions ParseQueryParams(HttpContext ctx)
+    {
+        var q = ctx.Request.Query;
+        var opts = new ConnzOptions();
+
+        if (q.TryGetValue("sort", out var sort))
+        {
+            opts.Sort = sort.ToString().ToLowerInvariant() switch
+            {
+                "cid" => SortOpt.ByCid,
+                "start" => SortOpt.ByStart,
+                "subs" => SortOpt.BySubs,
+                "pending" => SortOpt.ByPending,
+                "msgs_to" => SortOpt.ByMsgsTo,
+                "msgs_from" => SortOpt.ByMsgsFrom,
+                "bytes_to" => SortOpt.ByBytesTo,
+                "bytes_from" => SortOpt.ByBytesFrom,
+                "last" => SortOpt.ByLast,
+                "idle" => SortOpt.ByIdle,
+                "uptime" => SortOpt.ByUptime,
+                _ => SortOpt.ByCid,
+            };
+        }
+
+        if (q.TryGetValue("subs", out var subs))
+        {
+            if (subs == "detail")
+                opts.SubscriptionsDetail = true;
+            else
+                opts.Subscriptions = true;
+        }
+
+        if (q.TryGetValue("offset", out var offset) && int.TryParse(offset, out var o))
+            opts.Offset = o;
+
+        if (q.TryGetValue("limit", out var limit) && int.TryParse(limit, out var l))
+            opts.Limit = l;
+
+        return opts;
+    }
+
+    private static string FormatDuration(TimeSpan ts)
+    {
+        if (ts.TotalDays >= 1)
+            return $"{(int)ts.TotalDays}d{ts.Hours}h{ts.Minutes}m{ts.Seconds}s";
+        if (ts.TotalHours >= 1)
+            return $"{(int)ts.TotalHours}h{ts.Minutes}m{ts.Seconds}s";
+        if (ts.TotalMinutes >= 1)
+            return $"{(int)ts.TotalMinutes}m{ts.Seconds}s";
+        return $"{(int)ts.TotalSeconds}s";
+    }
+}
diff --git a/src/NATS.Server/Monitoring/MonitorServer.cs b/src/NATS.Server/Monitoring/MonitorServer.cs
index f33dbff..af23506 100644
--- a/src/NATS.Server/Monitoring/MonitorServer.cs
+++ b/src/NATS.Server/Monitoring/MonitorServer.cs
@@ -14,6 +14,7 @@ public sealed class MonitorServer : IAsyncDisposable
     private readonly WebApplication _app;
     private readonly ILogger<MonitorServer> _logger;
     private readonly VarzHandler _varzHandler;
+    private readonly ConnzHandler _connzHandler;
 
     public MonitorServer(NatsServer server, NatsOptions options, ServerStats stats, ILoggerFactory loggerFactory)
     {
@@ -27,6 +28,7 @@ public sealed class MonitorServer : IAsyncDisposable
         var basePath = options.MonitorBasePath ?? "";
 
         _varzHandler = new VarzHandler(server, options);
+        _connzHandler = new ConnzHandler(server);
 
         _app.MapGet(basePath + "/", () =>
         {
@@ -51,12 +53,13 @@ public sealed class MonitorServer : IAsyncDisposable
             return Results.Ok(await _varzHandler.HandleVarzAsync(ctx.RequestAborted));
         });
 
-        // Stubs for unimplemented endpoints
-        _app.MapGet(basePath + "/connz", () =>
+        _app.MapGet(basePath + "/connz", (HttpContext ctx) =>
         {
             stats.HttpReqStats.AddOrUpdate("/connz", 1, (_, v) => v + 1);
-            return Results.Ok(new { });
+            return Results.Ok(_connzHandler.HandleConnz(ctx));
         });
+
+        // Stubs for unimplemented endpoints
         _app.MapGet(basePath + "/routez", () =>
         {
             stats.HttpReqStats.AddOrUpdate("/routez", 1, (_, v) => v + 1);
diff --git a/tests/NATS.Server.Tests/MonitorTests.cs b/tests/NATS.Server.Tests/MonitorTests.cs
index 0d4c52e..898557a 100644
--- a/tests/NATS.Server.Tests/MonitorTests.cs
+++ b/tests/NATS.Server.Tests/MonitorTests.cs
@@ -98,6 +98,85 @@ public class MonitorTests : IAsyncLifetime
         varz.InBytes.ShouldBeGreaterThanOrEqualTo(5L);
     }
 
+    [Fact]
+    public async Task Connz_returns_connections()
+    {
+        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 {\"name\":\"test-client\",\"lang\":\"csharp\",\"version\":\"1.0\"}\r\n"u8.ToArray());
+        await Task.Delay(200);
+
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/connz");
+        response.StatusCode.ShouldBe(HttpStatusCode.OK);
+
+        var connz = await response.Content.ReadFromJsonAsync<Connz>();
+        connz.ShouldNotBeNull();
+        connz.NumConns.ShouldBeGreaterThanOrEqualTo(1);
+        connz.Conns.Length.ShouldBeGreaterThanOrEqualTo(1);
+
+        var conn = connz.Conns.First(c => c.Name == "test-client");
+        conn.Ip.ShouldNotBeNullOrEmpty();
+        conn.Port.ShouldBeGreaterThan(0);
+        conn.Lang.ShouldBe("csharp");
+        conn.Version.ShouldBe("1.0");
+        conn.Uptime.ShouldNotBeNullOrEmpty();
+    }
+
+    [Fact]
+    public async Task Connz_pagination()
+    {
+        var sockets = new List<Socket>();
+        try
+        {
+            for (int i = 0; i < 3; i++)
+            {
+                var s = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+                await s.ConnectAsync(new IPEndPoint(IPAddress.Loopback, _natsPort));
+                var ns = new NetworkStream(s);
+                var buf = new byte[4096];
+                _ = await ns.ReadAsync(buf);
+                await ns.WriteAsync("CONNECT {}\r\n"u8.ToArray());
+                sockets.Add(s);
+            }
+            await Task.Delay(200);
+
+            var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/connz?limit=2&offset=0");
+            var connz = await response.Content.ReadFromJsonAsync<Connz>();
+
+            connz!.Conns.Length.ShouldBe(2);
+            connz.Total.ShouldBeGreaterThanOrEqualTo(3);
+            connz.Limit.ShouldBe(2);
+            connz.Offset.ShouldBe(0);
+        }
+        finally
+        {
+            foreach (var s in sockets) s.Dispose();
+        }
+    }
+
+    [Fact]
+    public async Task Connz_with_subscriptions()
+    {
+        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\nSUB foo 1\r\nSUB bar 2\r\n"u8.ToArray());
+        await Task.Delay(200);
+
+        var response = await _http.GetAsync($"http://127.0.0.1:{_monitorPort}/connz?subs=true");
+        var connz = await response.Content.ReadFromJsonAsync<Connz>();
+
+        var conn = connz!.Conns.First(c => c.NumSubs >= 2);
+        conn.Subs.ShouldNotBeNull();
+        conn.Subs.ShouldContain("foo");
+        conn.Subs.ShouldContain("bar");
+    }
+
     private static int GetFreePort()
     {
         using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);