Costume Mario is the form takes after touching a in , allowing him to transform into numerous Nintendo and third-party characters, objects and groups or combinations thereof. Upon scanning certain onto the 's , an 8-bit version of the amiibo character (16-bit if a Sonic the Hedgehog amiibo is used) will be unlocked in the course maker, allowing the player to use it any time from then on without having to scan the amiibo again. In addition to amiibo characters, Mario can also transform into certain objects, such as a . Costumes can also be unlocked through the 100 Mario Challenge mode, including the characters which have amiibo unlock conditions, or by completing certain .
Costume Mario functions the same as , but with 's physical stature; although some of the costumes are slightly larger. While Mario wears a costume, he cannot wear any or collect a . In addition to appearance, various sound effects may also be changed to reflect the character, such as the sound effects and "Course Clear" fanfare. The power-up sound effect may also be replaced by a different sound effect from the character's home series. The non-DLC series costumes are notable for lacking special sound effects, instead using the standard sounds. Each character has a unique pose if the player presses and holds . The messages seen after finishing the 10 Mario Challenge and the 100 Mario Challenge also varies depending on the appearance of the player.