A method for preparing a positive electrode active material and a positive electrode active material prepared by the method are provided. The method includes preparing a lithium composite transition metal oxide represented by Formula 1, and washing the lithium composite transition metal oxide with a cleaning liquid containing cleaning water and a surfactant. The cleaning liquid contains cleaning water in an amount of no less than 50 parts by weight and less than 400 parts by weight based on 100 parts by weight of the lithium composite transition metal oxide.