diff --git a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyHandler.java b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyHandler.java
new file mode 100644
index 0000000..9a2b43b
--- /dev/null
+++ b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyHandler.java
@@ -0,0 +1,63 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby.LobbyId;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.lobby.LobbyManager;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.User;
+import ch.unibas.dmi.dbis.cs108.casono.server.domain.user.UserRegistry;
+import ch.unibas.dmi.dbis.cs108.casono.server.app.checks.UserLoggedInCheck;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.execution.CommandHandler;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.ErrorResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.OkResponse;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.response.dispatcher.ResponseDispatcher;
+
+public class JoinLobbyHandler extends CommandHandler<JoinLobbyRequest> {
+    private final LobbyManager lobbyManager;
+    private final UserRegistry userRegistry;
+
+    public JoinLobbyHandler(
+            ResponseDispatcher responseDispatcher,
+            LobbyManager lobbyManager,
+            UserRegistry userRegistry) {
+        super(responseDispatcher);
+        this.lobbyManager = lobbyManager;
+        this.userRegistry = userRegistry;
+        addCheck(new UserLoggedInCheck(userRegistry));
+    }
+
+    @Override
+    public void execute(JoinLobbyRequest request) {
+        LobbyId lid = LobbyId.of(request.getId());
+        var lobby = lobbyManager.getLobby(lid);
+        if (lobby == null) {
+            responseDispatcher.dispatch(
+                    new ErrorResponse(request.getContext(), "LOBBY_NOT_FOUND", "Lobby not found"));
+            return;
+        }
+
+        var maybeUser = userRegistry.getBySessionId(request.getContext().sessionId());
+        if (maybeUser.isEmpty()) {
+            // UserLoggedInCheck should normally prevent this; keep safe fallback
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "USER_NOT_LOGGED_IN",
+                            "No user associated with session"));
+            return;
+        }
+
+        User user = maybeUser.get();
+        String username = user.getName();
+
+        boolean ok = lobbyManager.addPlayerToLobby(username, lid);
+        if (!ok) {
+            responseDispatcher.dispatch(
+                    new ErrorResponse(
+                            request.getContext(),
+                            "LOBBY_FULL_OR_ALREADY_IN",
+                            "Lobby full or user already in lobby"));
+            return;
+        }
+
+        responseDispatcher.dispatch(new OkResponse(request.getContext()));
+    }
+}
diff --git a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyParser.java b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyParser.java
new file mode 100644
index 0000000..facb1a4
--- /dev/null
+++ b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyParser.java
@@ -0,0 +1,15 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.command.parsing.CommandParser;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.PrimitiveRequest;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.accessor.RequestParameterAccessor;
+
+public class JoinLobbyParser implements CommandParser<JoinLobbyRequest> {
+    @Override
+    public JoinLobbyRequest parse(PrimitiveRequest primitiveRequest) {
+        RequestParameterAccessor accessor =
+                new RequestParameterAccessor(primitiveRequest.parameters());
+        int id = accessor.require("ID", Integer::parseInt);
+        return new JoinLobbyRequest(primitiveRequest.context(), id);
+    }
+}
diff --git a/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyRequest.java b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyRequest.java
new file mode 100644
index 0000000..8c455f2
--- /dev/null
+++ b/src/main/java/ch/unibas/dmi/dbis/cs108/casono/server/app/commands/lobby/join_lobby/JoinLobbyRequest.java
@@ -0,0 +1,17 @@
+package ch.unibas.dmi.dbis.cs108.casono.server.app.commands.lobby.join_lobby;
+
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.Request;
+import ch.unibas.dmi.dbis.cs108.casono.server.network.protocol.request.RequestContext;
+
+public class JoinLobbyRequest extends Request {
+    private final int id;
+
+    public JoinLobbyRequest(RequestContext context, int id) {
+        super(context);
+        this.id = id;
+    }
+
+    public int getId() {
+        return id;
+    }
+}