Is There a 9th World in New Super Mario Bros. Wii? No – Only 8 Worlds + Star Courses

No, New Super Mario Bros. Wii has only 8 worlds. Each world contains multiple levels and a boss battle.

If you're looking for extra content, the game features secret "Star Courses" accessible by finding hidden exits or collecting Star Coins. There is no World 9.

You might be thinking of a different Mario game, or perhaps a secret level within the existing 8 worlds.