Acorn Plains is the first world of New Super Mario Bros. U. It is a plains world with trees, shrubs, and jagged mountains. The world contains the Acorn Tree, which is one of the sources of the Super Acorn power-up. The world lies south of Layer-Cake Desert, west of Sparkling Waters, and Soda Jungle, and south-west of Frosted Glacier.
The world contains eight levels: 5 normal levels, a Tower level, a Castle level, and a secret level. The world also includes a Piranha Plant Enemy Course and two Toad Houses, a Balloon Baby Yoshi also appears on the world map stood next to the spot which brings the player to the level Mushroom Heights.
- Acorn Plains-1: Acorn Plains Way
- Acorn Plains-2: Tilted Tunnel
- Acorn Plains-Tower: Crushing-Cogs Tower
- Acorn Plains-3: Yoshi Hill
- Acorn Plains-4: Mushroom Heights
- Acorn Plains-5: Rise of the Piranha Plants
- Acorn Plains-Castle: Lemmy's Swingback Castle
- Acorn Plains-Secret: Blooper's Secret Lair
- Acorn Plains-1: Waddlewing Warning!
- Acorn Plains-2: Crooked Cavern
- Acorn Plains-Tower: Flame Gear Tower
- Acorn Plains-3: Rolling Yoshi Hills
- Acorn Plains-4: Piranha Heights
- Acorn Plains-5: Piranha Gardens
- Acorn Plains-Castle: Lemmy's Lights-Out Castle
- Acorn Plains-Secret: Cheep Chomp Chase
- As shown in a few early builds of the game, the world's map looked substantially different. The map was significantly less detailed, the Acorn Trees did not show the Super Acorns, the mushroom platforms that leaded to Mushroom Heights were just red, the map had two red Toad Houses with the green Toad House in the spot where Balloon Baby Yoshi would've appeared in the retail game; there were two Enemy Courses instead of one and the spot where the Balloon Baby Yoshi is has been moved next to the level entrance leading to Mushroom Heights, Balloon Baby Yoshi also doesn't appear on the map at all as a green Toad House is in place of where he appears, this is likely because Balloon Baby Yoshis (along with Bubble Baby Yoshis) weren't supposed to appear on the world map and would've appeared in a Yoshi Egg hidden inside a block, although these two Baby Yoshi eggs go unused.