This positive electrode active material for a nonaqueous electrolyte secondary battery includes a lithium transition metal composite oxide that contains at least 80 mol % of Ni with respect to the total number of moles of metal elements excluding Li. B is present at least on the surfaces of the particles of the composite oxide. When particles having a particle diameter larger than a volume-based particle diameter of 70% (D70) are defined as first particles and particles having a particle diameter smaller than a volume-based particle diameter of 30% (D30) are defined as second particles, the molar fraction of B with respect to the total number of moles of metal elements excluding Li in the second particles is higher than the molar fraction of B with respect to the total number of moles of metal elements excluding Li in the first particles.