A piston seal device for a reciprocating compressor, having a piston slidably mounted in a cylinder bore and having a circumferential groove, and a resilient piston ring fitted in the circumferential groove of the piston and sealingly engageable with the cylinder bore. The circumferential groove includes a centrally recessed bottom wall for causing the piston ring to be resiliently and arcuately deformed to have a pair of opposite outer corner edges sealingly engageable with the cylinder bore. The piston sealing device thus provides a great sealing force both at the discharge stage and the intake stage, thereby improving the efficiency and lubrication of the compressor.