A piston pump includes a piston for increasing and decreasing the volume of a pump chamber formed in a cylinder bore of a cylinder member, by axially reciprocating movement thereof; a suction valve provided in the piston, and opening and closing when the volume of the pump chamber increases and decreases, respectively; and a discharge valve provided at a discharge side of the pump chamber, and opening and closing when the volume of the pump chamber decreases and increases, respectively. A plurality of mutually communicating, axially extending grooves are formed on an outer circumferential portion of the piston located on the pump-chamber side. A cylindrical seat member is fixedly and externally fitted to the piston in a region of the communicating grooves. A valve port and a valve seat of the suction valve are formed at an end portion of the seat member located on the pump-chamber side.