Successfully complete the game to unlock World 9. The levels in World 9 are only available if all Star Coins are collected in the previous Worlds.

Level 9-1: Collect all Star Coins in World 1.
Level 9-2: Collect all Star Coins in World 2.
Level 9-3: Collect all Star Coins in World 3.
Level 9-4: Collect all Star Coins in World 4.
Level 9-5: Collect all Star Coins in World 5.
Level 9-6: Collect all Star Coins in World 6.
Level 9-7: Collect all Star Coins in World 7.
Level 9-8: Collect all Star Coins in World 8.
Level 9-9: Collect all Star Coins in all previous levels of World 9.