Provided is a positive electrode active material for a non-aqueous electrolyte secondary battery, the active material including a lithium-transition metal composite oxide containing lithium, nickel, cobalt, and manganese, having a layered structure, having a ratio D50/DSEM of from 1 to 4, and having a ratio of a number of moles of nickel to a total number of moles of metals other than lithium of greater than 0.8 and less than 1, a ratio of a number of moles of cobalt to the total number of moles of metals other than lithium of less than 0.2, a ratio of a number of moles of manganese to the total number of moles of metals other than lithium of less than 0.2, and a ratio of the number of moles of manganese to a sum of the number of moles of cobalt and the number of moles of manganese of less than 0.58.