King Boo with his Bowser suit as seen in Luigi's Mansion.

King Boo (King Teresa in Japan) is a fictional character in Nintendo's Mario franchise. He first appears as the final boss of Luigi's Mansion and plays minor roles in various other Mario games, including the Mario Kart and Mario Party series. King Boo is a member of the Boo species, though he is larger than the average Boo and dons a crown, with a large ruby in Luigi's Mansion, and a regular crown in subsequent appearances, on his head. It is well distinguished by its blue eyes, green tongue, and unique senile laugh. As well as appearing as the main antagonist of Luigi's Mansion, King Boo also appears as a boss in Super Mario 64, Super Mario Sunshine, Mario Kart DS, and Super Princess Peach. He is also a playable character in Mario Superstar Baseball, Mario Super Sluggers, and Mario Kart Wii.

In his first appearance, he fights using a magical Bowser suit, which enables him to suck in mass amounts of air, jump incredibly high, breathe fire and ice, and materialize bombs out of thin air.