Provided is a positive electrode active material for a lithium ion secondary battery which combines high capacity with favorable charging/discharging cycle characteristics. In this positive electrode active material for a lithium ion secondary battery, the average value of the particle diameters of the primary particles evaluated with a scanning electron microscope is 50-550 nm, and the R value is greater than 0.420 but less than or equal to 0.460, the R value being calculated by formula (2) as follows: (2) R value=(I006+I102)/I101 on the basis of the intensity I006 calculated from the height of a peak identified as the 006-plane of an α-NaFeO2 type layered structure in X-ray powder diffraction measurement, the intensity I102 calculated from the height of a peak identified as the 102-plane, and the intensity I101 calculated from the height of a peak identified as the 101-plane.