Like many characters, Bowser also has alternate forms. So far Bowser has only been seen in one.
- Metal Bowser: This is the form Bowser takes on when he swallows a Metal Mushroom. Bowser entered this form while fighting Mario during Episode 1. In this form Bowser is covered in metal, making him nearly invincible. Metal Bowser is far stronger than normal Bowser, especially with respect to his fire attacks. This metallic state acts more like a suit of armor than an actual transformation, though, as Bowser reverted to normal after his steely hide cracked and fell apart from an impact by the heavy capsule that bore Sonic and Shadow.
- Giga Bowser: A Super Smash Bros. series exclusive transformation that served as a hidden final boss for Adventure mode in Melee and a Final Smash in Brawl. In this form, Bowser triples in size, with his strength and endurance increasing an equal amount. He also gains a freezing element to his basic attacks and a far more imposing appearance. It is generally assumed that Bowser would become Giga Bowser if he possessed all the Chaos Emeralds.
- Shrowser: In Mario and Luigi: Partners in Time, Bowser ate the remains of Princess Shroob and her spirit possessed him. His speed and power increase greatly, and he's so fast that the player cannot attack but simply return the attacks sent by Shrowser.
- Bowlletta: In Mario and Luigi: Superstar Saga, Bowser was possessed by Cackletta while she was looking for another body. Bowser's powers were combined with hers to fight Mario and Luigi.