The Master Key is a key item that appears in Wario: Master of Disguise.
The Master Key is a large key that is yellow in color and has a pentagon-shaped end.
The Master Key can be found on board the S.S. Caviar where it is located inside of a purple treasure chest in the upper-right room of the area. The Master Key can then be used to unlock doors. After unlocking one door, the number "2" changes to a "1" implying it may be a set of keys instead (despite Wario describing it's one).