According to one embodiment, a positive electrode active material for a non-aqueous electrolyte secondary battery contains a lithium/transition metal composite oxide that contains 80 mol % or more, relative to the total mol number of metal elements other than Li, of Ni and at least one kind of metal element selected from among Co, Mn, Al, W, Mg, Mo, Nb, Ti, Si and Zr. When a filtrate of a suspension, said suspension being prepared by adding 250 mg of the positive electrode active material to 10 mL of a 17.5 mass % aqueous solution of hydrochloric acid, dissolving by heating at 90° C. for 2 hours and then diluting to 50 mL, is analyzed by inductively coupled plasma mass spectrometry, the elution amount of S in the filtrate is 0.002 mmol or greater.