Disclosed are a rotor for an electric motor, an electric motor, and a compressor. The rotor includes: a rotor core (1), provided with a shaft hole (11) and four magnet grooves (12); and four permanent magnets (2), disposed in corresponding magnet grooves, each permanent magnet including a first surface (21) and a second surface (22). When viewed from a projection surface, the first surface has an arc segment (211), and the second surface has a plurality of straight segments connected in sequence. A thickness H of each permanent magnet on a corresponding magnetic pole centerline, a length L of the second surface projected on the projection surface, and a radius R of the rotor core satisfying the following relationships: 0.24≤H/R≤0.26 and 1.07≤L/R≤1.11.