The present disclosure is directed to a positive electrode active material for non-aqueous electrolyte secondary batteries that is capable of suppressing an increase in battery direct current resistance due to high-temperature storage (e.g., storage at 60° C. or higher). Positive electrode active material particles in one aspect of the present disclosure include secondary particles formed by aggregation of primary particles of a lithium transition metal oxide containing Ni and Mn and include a boron compound present in the inner part and surface of the secondary particles. The difference in composition ratio between Ni and Mn in the lithium transition metal oxide is more than 0.2. The proportion of the boron element content in the inner part of the secondary particles to the total boron element content in the inner part and surface of the secondary particles is in the range from 5% by mass to 60% by mass.