Provided are a positive electrode active material for a sodium ion secondary battery excellent in quick charge-discharge characteristics by preventing fusion between particles of powder during firing, and a method of producing the same. A positive electrode active material for a sodium ion secondary battery of the present invention includes inorganic powder that contains at least one kind of transition metal element selected from the group consisting of Cr, Fe, Mn, Co, and Ni, Na, P, and O, and has a BET specific surface area of from 3 m2/g to 50 m2/g.