diff --git a/Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs b/Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs
index 33627e9..22631df 100644
--- a/Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs
+++ b/Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs
@@ -106,9 +106,11 @@ namespace MQTTnet.Server.Internal
                     return;
                 }
 
-                clientConnection = await CreateClientConnection(connectPacket, channelAdapter, connectionValidatorContext.SessionItems).ConfigureAwait(false);
-
                 connAckPacket = channelAdapter.PacketFormatterAdapter.DataConverter.CreateConnAckPacket(connectionValidatorContext);
+
+                // Pass connAckPacket so that IsSessionPresent flag can be set if the client session already exists
+                clientConnection = await CreateClientConnection(connectPacket, connAckPacket, channelAdapter, connectionValidatorContext.SessionItems).ConfigureAwait(false);
+
                 await channelAdapter.SendPacketAsync(connAckPacket, cancellationToken).ConfigureAwait(false);
 
                 await _eventDispatcher.SafeNotifyClientConnectedAsync(connectPacket, channelAdapter).ConfigureAwait(false);
@@ -450,7 +452,8 @@ namespace MQTTnet.Server.Internal
         }
 
         async Task<MqttClientConnection> CreateClientConnection(
-            MqttConnectPacket connectPacket, 
+            MqttConnectPacket connectPacket,
+            MqttConnAckPacket connAckPacket,
             IMqttChannelAdapter channelAdapter, 
             IDictionary<object, object> sessionItems)
         {
@@ -476,6 +479,7 @@ namespace MQTTnet.Server.Internal
                         else
                         {
                             _logger.Verbose("Reusing existing session of client '{0}'.", connectPacket.ClientId);
+                            connAckPacket.IsSessionPresent = true;
                         }
                     }
 
diff --git a/Tests/MQTTnet.Core.Tests/MQTTv5/Server_Tests.cs b/Tests/MQTTnet.Core.Tests/MQTTv5/Server_Tests.cs
index be589d5..18fde51 100644
--- a/Tests/MQTTnet.Core.Tests/MQTTv5/Server_Tests.cs
+++ b/Tests/MQTTnet.Core.Tests/MQTTv5/Server_Tests.cs
@@ -1,4 +1,4 @@
-using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Client;
 using MQTTnet.Client.Options;
 using MQTTnet.Formatter;
@@ -38,5 +38,57 @@ namespace MQTTnet.Tests.MQTTv5
                 Assert.AreEqual(1, receivedMessagesCount);
             }
         }
+
+        [TestMethod]
+        public async Task Validate_IsSessionPresent()
+        {
+            using (var testEnvironment = new TestEnvironment(TestContext))
+            {
+                // Create server with persistent sessions enabled
+
+                await testEnvironment.StartServer(o => o.WithPersistentSessions());
+
+                const string ClientId = "Client1";
+
+                // Create client without clean session and long session expiry interval
+
+                var client1 = await testEnvironment.ConnectClient(o => o
+                    .WithProtocolVersion(MqttProtocolVersion.V500)
+                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                    .WithSessionExpiryInterval(9999)
+                    .WithCleanSession(false)
+                    .WithClientId(ClientId)
+                    .Build()
+                );
+
+                // Disconnect; empty session should remain on server
+
+                await client1.DisconnectAsync();
+
+                // Simulate some time delay between connections
+
+                await Task.Delay(1000);
+
+                // Reconnect the same client ID to existing session
+
+                var client2 = testEnvironment.CreateClient();
+                var options = testEnvironment.Factory.CreateClientOptionsBuilder()
+                    .WithProtocolVersion(MqttProtocolVersion.V500)
+                    .WithTcpServer("127.0.0.1", testEnvironment.ServerPort)
+                    .WithSessionExpiryInterval(9999)
+                    .WithCleanSession(false)
+                    .WithClientId(ClientId)
+                    .Build();
+
+
+                var result = await client2.ConnectAsync(options).ConfigureAwait(false);
+
+                await client2.DisconnectAsync();
+
+                // Session should be present
+
+                Assert.IsTrue(result.IsSessionPresent, "Session not present");
+            }
+        }
     }
 }