Bouldergeist is a ghost with the power to control rocks in the Ghostly Galaxy. He holds one of the Power Stars and was the one who presumably captured Luigi. He reappears in the sequel, Super Mario Galaxy 2 as one of the bosses faced in the Boss Blitz Galaxy, presumably to get his revenge.
He is a large ghost who is entirely made out of boulders. The inside of his body is a black swarm of smoke.
In his second phase, he grows two large boulder hands.
Bouldergeist makes his lair on the Bouldergeist Battle Planet. When Mario makes his way here and encounters him, Bouldergeist assembles his stone armour suit and strikes.
Bouldergeist's main attack is throwing rocks, boulders, and stones at Mario. They are quite easy to avoid if the player runs in a straight line. However, he'll occasionally summon a straight line of rocks, which hurts Mario if he's caught in it and restricts Mario's moving space. To defeat Bouldergeist, Mario must use his Star Spin to get the Bomb Boos produced from the black rocks the monster throws. After three hits, he transforms into his true form: a black ghost. Then he'll try to run, spawning Bomb Boos in his wake. The player must use the Bomb Boos to hit him. In his second phase, the monster grows rock hands. These rock hands give him new attacks such as slamming them into the ground or trying to punch Mario. Plus, the rock geyser attack makes two lines instead of one. Simply apply the first strategy, but the right hand would restrict you from hitting the body. After Bouldergeist loses his stamina, the ghost's core explodes, and Mario can get his hands on the Power Star dropped by him. Bouldergeist retains his strategy in Super Mario Galaxy 2's Boss Blitz Galaxy.
- Bouldergeist is a portmanteau of the word "Boulder" and "Poltergeist."
- Bouldergeist is very similar to Thardus in Metroid Prime.