Bowser, also known as King Koopa is the boss of the World 1 Castle. Mario defeats him, dropping him into lava. He then turns into Dry Bowser, and Bowser Jr. takes him to World 8. However, Mario defeats him once again by dropping him into a pit. Then, Bowser Jr. turns Bowser back to normal in Bowser's Castle, and Bowser Jr. and Bowser fight Mario at the same time.
- In the fights in World 8, Mario must wait for Bowser to do a "high jump" and then dash under him so he can hit the switch. This is because there isn't a platform.
- Unlike the rest of the bosses, you can't jump on him.