One Piece: Chopper's Big Adventure is a Japanese role-playing game for the Wonderswan handheld featuring characters from the popular anime and manga series One Piece. It was developed by Bandai and released on October 16, 2003 by Bandai Namco.

One day, the Straw Hat Pirates travel to a mysterious island. While the Straw Hats explore around the island, they all suddenly turn into animals; Luffy into a monkey, Zoro into a tiger, Sanji into a fox, Nami into a cat, Usopp into a kappa, and Robin into a parrot. Only Chopper remains as he was.

It is up to Chopper to not only help the crew to turn back to their original state, but to also figure out the cause of their mysterious transformation.