Petey Piranha appears as the boss of Rumble Volcano in Paper Mario: Sticker Star. This is his debut in the Paper Mario series. Like other bosses, he gains power from a Royal Sticker that he took and ate from a tribe of Spear Guys who described him as gluttonous. Upon first sighting Mario and Kersti, he swallows Kersti whole, forcing Mario to go after him. Presumably during this time, he also ate several Koopas and stickers. Once found, he immediately attacks Mario. He mainly fights by flying with his leaves before slamming down on Mario. He can also spit out a ball of goo at Mario which can sometimes cause him to gain the soggy status effect. After losing HP, he uses his leaves to spin around rapidly to attack Mario but doing so makes him dizzy. Sometimes, Mario gets dizzy if he is hit by this attack. If he is attacked in this state, his weak point (his belly button) becomes exposed, which makes him vulnerable to serious damage. Once he is attacked he spits out Kersti and a Dry Bones. After he is defeated, he disappears and gives up the Royal Sticker. Petey has 300 HP. When he has 100 HP left, he begins a more powerful attack when he bites Mario 3 times. Each time he bites Mario, he deals 7 damage.
It is notable that this is the first game where Petey actually speaks normally (albeit he speaks only one sentence).