diff --git a/src/Microsoft.Net.WebSockets/Constants.cs b/src/Microsoft.Net.WebSockets/Constants.cs
index 8fb002913e..6877c70d0b 100644
--- a/src/Microsoft.Net.WebSockets/Constants.cs
+++ b/src/Microsoft.Net.WebSockets/Constants.cs
@@ -10,7 +10,8 @@ namespace Microsoft.Net.WebSockets
     {
         public static class Headers
         {
-            public const string WebSocketVersion = "Sec-WebSocket-Version";
+            public const string SecWebSocketVersion = "Sec-WebSocket-Version";
+            public const string SecWebSocketProtocol = "Sec-WebSocket-Protocol";
             public const string SupportedVersion = "13";
         }
 
diff --git a/src/Microsoft.Net.WebSockets/WebSocketClient.cs b/src/Microsoft.Net.WebSockets/WebSocketClient.cs
index 60636a05f0..ca77944110 100644
--- a/src/Microsoft.Net.WebSockets/WebSocketClient.cs
+++ b/src/Microsoft.Net.WebSockets/WebSocketClient.cs
@@ -1,5 +1,7 @@
 using System;
+using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Net;
 using System.Net.WebSockets;
 using System.Threading;
@@ -26,6 +28,13 @@ namespace Microsoft.Net.WebSockets.Client
         {
             ReceiveBufferSize = 1024 * 16;
             KeepAliveInterval = TimeSpan.FromMinutes(2);
+            SubProtocols = new List<string>();
+        }
+
+        public IList<string> SubProtocols
+        {
+            get;
+            private set;
         }
 
         public TimeSpan KeepAliveInterval
@@ -64,8 +73,11 @@ namespace Microsoft.Net.WebSockets.Client
 
             CancellationTokenRegistration cancellation = cancellationToken.Register(() => request.Abort());
 
-            request.Headers[Constants.Headers.WebSocketVersion] = Constants.Headers.SupportedVersion;
-            // TODO: Sub-protocols
+            request.Headers[Constants.Headers.SecWebSocketVersion] = Constants.Headers.SupportedVersion;
+            if (SubProtocols.Count > 0)
+            {
+                request.Headers[Constants.Headers.SecWebSocketProtocol] = string.Join(", ", SubProtocols);
+            }
 
             if (ConfigureRequest != null)
             {
@@ -85,14 +97,19 @@ namespace Microsoft.Net.WebSockets.Client
             if (response.StatusCode != HttpStatusCode.SwitchingProtocols)
             {
                 response.Dispose();
-                throw new InvalidOperationException("Incomplete handshake");
+                throw new InvalidOperationException("Incomplete handshake, invalid status code: " + response.StatusCode);
             }
+            // TODO: Validate Sec-WebSocket-Key/Sec-WebSocket-Accept
 
-            // TODO: Sub protocol
+            string subProtocol = response.Headers[Constants.Headers.SecWebSocketProtocol];
+            if (!string.IsNullOrEmpty(subProtocol) && !SubProtocols.Contains(subProtocol, StringComparer.OrdinalIgnoreCase))
+            {
+                throw new InvalidOperationException("Incomplete handshake, the server specified an unknown sub-protocol: " + subProtocol);
+            }
 
             Stream stream = response.GetResponseStream();
 
-            return CommonWebSocket.CreateClientWebSocket(stream, null, KeepAliveInterval, ReceiveBufferSize, useZeroMask: UseZeroMask);
+            return CommonWebSocket.CreateClientWebSocket(stream, subProtocol, KeepAliveInterval, ReceiveBufferSize, useZeroMask: UseZeroMask);
         }
     }
 }
diff --git a/test/Microsoft.Net.WebSockets.Test/WebSocketClientTests.cs b/test/Microsoft.Net.WebSockets.Test/WebSocketClientTests.cs
index 91d1045e4c..f4c6b207fe 100644
--- a/test/Microsoft.Net.WebSockets.Test/WebSocketClientTests.cs
+++ b/test/Microsoft.Net.WebSockets.Test/WebSocketClientTests.cs
@@ -35,6 +35,32 @@ namespace Microsoft.Net.WebSockets.Test
             }
         }
 
+        [Fact]
+        public async Task NegotiateSubProtocol_Success()
+        {
+            using (HttpListener listener = new HttpListener())
+            {
+                listener.Prefixes.Add(ServerAddress);
+                listener.Start();
+                Task<HttpListenerContext> serverAccept = listener.GetContextAsync();
+
+                WebSocketClient client = new WebSocketClient();
+                client.SubProtocols.Add("alpha");
+                client.SubProtocols.Add("bravo");
+                client.SubProtocols.Add("charlie");
+                Task<WebSocket> clientConnect = client.ConnectAsync(new Uri(ClientAddress), CancellationToken.None);
+
+                HttpListenerContext serverContext = await serverAccept;
+                Assert.True(serverContext.Request.IsWebSocketRequest);
+                Assert.Equal("alpha, bravo, charlie", serverContext.Request.Headers["Sec-WebSocket-Protocol"]);
+                HttpListenerWebSocketContext serverWebSocketContext = await serverContext.AcceptWebSocketAsync("Bravo");
+
+                WebSocket clientSocket = await clientConnect;
+                Assert.Equal("Bravo", clientSocket.SubProtocol);
+                clientSocket.Dispose();
+            }
+        }
+
         [Fact]
         public async Task SendShortData_Success()
         {