The present invention provides a positive electrode active material for a secondary battery, which includes a core, a shell disposed to surround the core, and a buffer layer which is disposed between the core and the shell and includes pores and a three-dimensional network structure connecting the core and the shell, wherein, the core, the shell, and the three-dimensional network structure of the buffer layer each independently include a lithium nickel manganese cobalt-based composite metal oxide and at least one metallic element of the nickel, the manganese, and the cobalt has a concentration gradient that gradually changes in any one region of the core, the shell, and the entire positive electrode active material.