Shadow Mario is one of the primary antagonists in Super Mario Sunshine. In reality, he is just Bowser Jr. disguised as Mario so that his opponent would be blamed for his wrongdoings. In the game, Shadow Mario would spread toxic goop across all of Isle Delfino, which in turn would cause all of the Shine Sprites to vanish. His plan works and Mario gets blamed, and is then ordered by a local judge to clean up all of the paint.
On multiple occasions, Mario will be required to hunt down Shadow Mario by spraying him with water, which is apparently is his weak point. He'll do this quite often on every stage, and in the game's hub world when he kidnaps Princess Peach and a lone Yoshi Egg. Shadow Mario also appeared as a playable character in the game Mario Golf: Toadstool Tour.
In Super Mario Galaxy, a minor character called Cosmic Mario appeared who looked strikingly similar to Shadow Mario. He only appears in four galaxies only when the cosmic comet is in orbit and only then he races against you to the Star.
Super Smash Bros.
Super Smash Bros. for Nintendo 3DS/Wii U
Shadow Mario is featured in Super Smash Bros. for Nintendo 3DS/Wii U as Bowser Jr.'s Final Smash. Bowser Jr. transforms into Shadow Mario and paint a giant orange X on the screen, anyone in the X's hitbox will be damaged and pulled into the center of the X. After some time, the X will explode and cause additional damage to anyone in range.
- NA: In Super Mario Sunshine, someone went around causing havoc disguised as Mario. It turned out not to be the brawny Bowser, but his brainier son, Bowser Jr. Now he becomes Shadow Mario again for his Final Smash, painting a giant X on the screen. It sticks around after Bowser Jr. transforms back, letting him gleefully knock foes into it.
- EU: In Super Mario Sunshine, someone went around causing havoc disguised as Mario. It turned out not to be the brawny Bowser, but his brainier son, Bowser Jr. Now he becomes Shadow Mario again for his Final Smash, painting a giant cross on the screen. It sticks around after Bowser Jr. turns back, so he can gleefully knock people into it.