A method for producing a positive electrode active material process which can minimize deterioration, occurring in a washing process, on the surface of a positive electrode active material and effectively control residual lithium. The method includes (A) preparing a lithium transition metal oxide; and (B) mixing the lithium transition metal oxide and a washing solution, washing the mixture, and then drying the mixture, wherein the washing solution contains a phosphorus-based compound and is a basic solution, and the phosphorus-based compound is contained in the washing solution such that the amount of phosphorus is 100-1,000 ppm in weight based on the lithium transition metal oxide, and to a positive electrode active material produced by the method.