A positive electrode active material for a lithium ion secondary battery contains: a first compound represented by chemical formula Lix(NiyMa1-y)O2 (0.95≦x≦1.05, 0.70≦y≦0.95, where Ma is at least one element selected from Co, Mn, V, Ti, Fe, Zr, Nb, Mo, Al, and W); and a second compound represented by chemical formula LiVOPO4. W>5.0° C., where W is a full width at half maximum of an exothermic peak obtained between 150° C. and 260° C. by differential scanning calorimetry (DSC) performed on a mixture of the first compound and the second compound under a condition of 5° C./min.