The present invention relates to a positive electrode active material and a lithium secondary battery including the same, and more particularly, to a positive electrode active material including an overlithiated lithium manganese-based oxide, wherein through surface modification of the lithium manganese-based oxide, the surface resistance of the lithium manganese-based oxide is reduced, thereby improving the lifetime characteristics of the lithium secondary battery, which uses the lithium manganese-based oxide as a positive electrode active material, and a lithium secondary battery including the same.

A manganese-based solid solution positive-electrode material, wherein the manganese-based solid solution positive-electrode material has a layered structure, and a chemical formula of the manganese-based solid solution positive-electrode material is aNa.sub.2Mn.sub.xR.sub.1-xO.sub.3.Math.(1a)LiMn.sub.yM.sub.1-O.sub.2, where 0.05a<1, 0<x1, 0.1y1, and each of the R and the M in the chemical formula independently comprises any one or combination of at least two of: alkali metal elements, alkaline earth metal elements, and transition metal elements.