Abstract: The positive electrode active material for secondary batteries having a layered structure containing at least nickel, cobalt, and manganese, as single-crystal particles and/or secondary particles that are aggregates of a plurality of primary particles, wherein: an average particle strength of particles having a particle size of (D50)±1.0 ?m is 200 MPa or more, wherein (D50) is a particle size at a cumulative volume percentage of 50% by volume; and ?/? is set to satisfy 0.97??/??1.25, provided that ? is a full width at half maximum of a lower angle peak among two diffraction peaks appearing in a range of 2?=64.5±1° in an X-ray diffraction pattern, and ? is a full width at half maximum of a higher angle peak among the diffraction peaks.