In Mario Party 9, players vie to become the Superstar as they make their way through adventurous new stages, overcoming obstacles and collecting Mini Stars. Players also travel across each stage together in vehicles and join forces in boss battles to defeat a common enemy. They must work together to beat classic villains like Bowser and King Boo while still giving their individual scores a boost. Players take turns acting as Captain of the vehicle, hitting Dice Blocks to move the party toward the goal. As they progress, players may encounter 80 new minigames that might have them making pizzas, escaping haunted houses, bowling shells at Goombas and more.
In addition to collecting Mini Stars, players also earn Party Points that can be taken to the in-game Museum to unlock extras, such as character constellations, additional vehicles, a Donkey Kong-themed stage and more. There are also minigame modes that let players compete in shorter amounts of time, plus a Free Play mode for practicing individual minigames and a fun Extras mode. Additional Wii Remote controllers are required for all multiplayer modes, which support up to four players.