diff --git a/game/lobby.go b/game/lobby.go
index af03b81..d278f40 100644
--- a/game/lobby.go
+++ b/game/lobby.go
@@ -78,6 +78,14 @@ func (l *Lobby) ListRooms() []*LobbyRoom {
 	return result
 }
 
+func (l *Lobby) StartRoom(code string) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	if room, ok := l.rooms[code]; ok {
+		room.Status = RoomPlaying
+	}
+}
+
 func (l *Lobby) RemoveRoom(code string) {
 	l.mu.Lock()
 	defer l.mu.Unlock()
diff --git a/game/lobby_test.go b/game/lobby_test.go
index 25b6b7d..5a60040 100644
--- a/game/lobby_test.go
+++ b/game/lobby_test.go
@@ -27,6 +27,25 @@ func TestJoinRoom(t *testing.T) {
 	}
 }
 
+func TestRoomStatusTransition(t *testing.T) {
+	l := NewLobby()
+	code := l.CreateRoom("Test")
+	l.JoinRoom(code, "Alice")
+	r := l.GetRoom(code)
+	if r.Status != RoomWaiting {
+		t.Errorf("new room should be Waiting, got %d", r.Status)
+	}
+	l.StartRoom(code)
+	r = l.GetRoom(code)
+	if r.Status != RoomPlaying {
+		t.Errorf("started room should be Playing, got %d", r.Status)
+	}
+	err := l.JoinRoom(code, "Bob")
+	if err == nil {
+		t.Error("should not be able to join a Playing room")
+	}
+}
+
 func TestJoinRoomFull(t *testing.T) {
 	lobby := NewLobby()
 	code := lobby.CreateRoom("Test Room")
diff --git a/ui/model.go b/ui/model.go
index 53eb255..7ef6175 100644
--- a/ui/model.go
+++ b/ui/model.go
@@ -262,6 +262,7 @@ func (m Model) updateClassSelect(msg tea.Msg) (tea.Model, tea.Cmd) {
 					player.Fingerprint = m.fingerprint
 					m.session.AddPlayer(player)
 					m.session.StartGame()
+					m.lobby.StartRoom(m.roomCode)
 					m.gameState = m.session.GetState()
 					m.screen = screenGame
 				}