A rotor with permanent magnets for a rotary electric machine including a plurality of alternating north poles and south poles and made up of a plurality of permanent magnets arranged in first recesses extending axially and distributed regularly between a circumferential portion and a central portion of the magnetic mass of the rotor such as to define a plurality of circumferential polar sections, the first recesses being separated by central tabs with a thickness E, the permanent magnets having a polygonal radial section that comprises a substantially rectangular portion next to the circumferential portion adjacent to a substantially trapezoidal portion next to the central portion. The rotor having a ratio R0 (h/E) of a first height h of the trapezoidal portion to the thickness E of a central tab is predetermined such as to maximize the efficiency of the electric machine.