The invention aims to realize a clutch mechanism and a piston capable of preventing bubbles from staying in a piston chamber. The clutch mechanism (1) switches a clutch (7,8) to one of an engaging state and a released state by a piston (9) operated by an oil pressure. The piston (9) includes a first side surface portion (9A) configured to press the clutch (7,8), and a second side surface portion (9B) on which the oil pressure acts. The second side surface portion (9B) has an uneven shape in a circumferential direction.