Provided are a lithium manganese oxide positive active material for a lithium ion secondary battery and a lithium ion secondary battery including the same. The lithium manganese oxide positive active material includes a spinel lithium manganese oxide of three or more types of particles having different sizes mixed therein, wherein first type particles have an average diameter of 5 μm or greater, second type particles have an average diameter of 1 μm or less, third type particles have an average diameter of 200 nm or less, and the average diameter of the second type particles is greater than that of the third type particles.