diff --git a/OpenRA.Game/Server/Server.cs b/OpenRA.Game/Server/Server.cs
index fb33763952..2c6cdcced5 100644
--- a/OpenRA.Game/Server/Server.cs
+++ b/OpenRA.Game/Server/Server.cs
@@ -374,6 +374,9 @@ namespace OpenRA.Server
 						SendOrderTo(newConn, "Message", motd);
 				}
 
+				if (Map.RuleDefinitions.Any() && !LobbyInfo.IsSinglePlayer)
+					SendOrderTo(newConn, "Message", "This map contains custom rules. Game experience may change.");
+
 				if (handshake.Mod == "{DEV_VERSION}")
 					SendMessage("{0} is running an unversioned development build, ".F(client.Name) +
 					"and may desynchronize the game state if they have incompatible rules.");
diff --git a/OpenRA.Mods.Common/ServerTraits/LobbyCommands.cs b/OpenRA.Mods.Common/ServerTraits/LobbyCommands.cs
index 2a1c2304e1..2867ff62c3 100644
--- a/OpenRA.Mods.Common/ServerTraits/LobbyCommands.cs
+++ b/OpenRA.Mods.Common/ServerTraits/LobbyCommands.cs
@@ -371,6 +371,9 @@ namespace OpenRA.Mods.Common.Server
 
 						server.SendMessage("{0} changed the map to {1}.".F(client.Name, server.Map.Title));
 
+						if (server.Map.RuleDefinitions.Any())
+							server.SendMessage("This map contains custom rules. Game experience may change.");
+
 						return true;
 					}
 				},