A positive electrode active material includes a plurality of groups of particles. The plurality of groups of particles has a particle diameter of more than or equal to 300 nm and less than or equal to 3 μm. Each of the groups includes two or more particles. The two or more particles are each a lithium-containing complex phosphate including one or more of iron, nickel, manganese, and cobalt. The group of particles includes a first particle and a second particle each having a major diameter and a minor diameter in the upper surface when seen from a predetermined direction. The major diameters of the first and second particles are substantially parallel to each other. The major diameter of the first particle is two to six times larger than the minor diameter of the first particle and the minor diameter of the first particle is more than or equal to 20 nm and less than or equal to 130 nm.