Provided are a positive electrode active material capable of suppressing the reduction in capacity of a battery and the generation of gas during storage at high temperature in a charged state and a nonaqueous electrolyte secondary battery including the positive electrode active material. A positive electrode active material particle (20) includes a lithium transition metal oxide particle (21) containing a halogen atom and rare-earth compound particles (22) attached to the surface of the lithium transition metal oxide particle (21). The amount of the halogen atom present on the surface of the lithium transition metal oxide particle (21) is 5 mass percent or less of the total amount of the halogen atom contained in the lithium transition metal oxide particle (21). A rare-earth element making up the rare-earth compound particles (22) is one other than yttrium and scandium.