A piston for an automatic transmission, wherein a first piston member forming a cylindrical portion of the piston has a plurality of axial cutouts and a circumferential groove formed in one axial end portion thereof, and a second piston member forming a bottom portion of the piston has a plurality of radially extending engaging jaws fitted in the plurality of axial cutouts, while a retainer ring is fitted in the circumferential groove and held in abutting contact with the engaging jaws fitted in the axial cutouts, whereby the first and second piston members are fixed to each other. At least one of the engaging jaws of the second piston member cooperates with the retainer ring to form a gap therebetween, which prevents a stress concentration on portions of the circumferential groove adjacent to the axial cutouts, upon an axial movement of the piston, so that the required thickness of the retainer ring can be reduced, leading to size reduction of the piston.