A positive electrode active material for a nonaqueous electrolyte secondary battery includes particles of a lithium-transition metal composite oxide that contains nickel in the composition thereof and has a layered structure. The particles have an average particle size DSEM based on electron microscopic observation in a range of 1 μm to 7 μm in which a ratio D50/DSEM of a 50% particle size D50 in volume-based cumulative particle size distribution to the average particle size based on electron microscopic observation is in a range of 1 to 4, and a ratio D90/D10 of a 90% particle size D90 to a 10% particle size D10 in volume-based cumulative particle size distribution is 4 or less.