The present invention relates to a positive electrode active material for a secondary battery, which includes a core including a lithium composite metal oxide, and a surface treatment layer which is disposed on the core and includes an amorphous oxide containing a lithium (Li) oxide, a boron (B) oxide, and an aluminum (Al) oxide, wherein an amount of a lithium by-product present on a surface of the positive electrode active material is less than 0.55 wt % based on a total weight of the positive electrode active material, and a method of preparing the same.