A positive electrode active material for a secondary battery, the positive electrode active material comprising: a core including a lithium composite metal oxide represented by Formula 1 <Formula 1> Li a (Ni x Co y M1 z) b M2 c O 2 wherein, in Formula 1, M1 is at least one element selected from the group consisting of manganese and aluminum, and M2 is at least one element selected from the group consisting of barium, calcium, zirconium, titanium, magnesium, tantalum, niobium, and molybdenum, wherein 1≤a≤1.5, 0.9≤b≤1, 0≤c≤0.1, 0.6≤x<1, 0<y<0.4, 0<z≤0.4, and b+c=1; and a surface treatment layer which is disposed on the core and includes an amorphous oxide containing a lithium oxide, a boron oxide, and an aluminum oxide, wherein the lithium oxide, the boron oxide, and the aluminum oxide are chemically bonded to one another and wherein the lithium oxide is Li 2 O, the boron oxide is B 2 O 3, and the aluminum oxide is Al 2 O 3; and in the amorphous oxide in the surface treatment layer, an amount of the aluminum oxide is greater than an amount of the boron oxide, and an amount of a lithium by-product comprising at least one of LiOH or Li 2 CO 3 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. a core including a lithium composite metal oxide represented by Formula 1 <Formula 1> Li a (Ni x Co y M1 z) b M2 c O 2 wherein, in Formula 1, M1 is at least one element selected from the group consisting of manganese and aluminum, and M2 is at least one element selected from the group consisting of barium, calcium, zirconium, titanium, magnesium, tantalum, niobium, and molybdenum, wherein 1≤a≤1.5, 0.9≤b≤1, 0≤c≤0.1, 0.6≤x<1, 0<y<0.4, 0<z≤0.4, and b+c=1; and a surface treatment layer which is disposed on the core and includes an amorphous oxide containing a lithium oxide, a boron oxide, and an aluminum oxide, wherein the lithium oxide, the boron oxide, and the aluminum oxide are chemically bonded to one another and wherein the lithium oxide is Li 2 O, the boron oxide is B 2 O 3, and the aluminum oxide is Al 2 O 3; and in the amorphous oxide in the surface treatment layer, an amount of the aluminum oxide is greater than an amount of the boron oxide, and an amount of a lithium by-product comprising at least one of LiOH or Li 2 CO 3 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. M1 is at least one element selected from the group consisting of manganese and aluminum, and M2 is at least one element selected from the group consisting of barium, calcium, zirconium, titanium, magnesium, tantalum, niobium, and molybdenum, wherein 1≤a≤1.5, 0.9≤b≤1, 0≤c≤0.1, 0.6≤x<1, 0<y<0.4, 0<z≤0.4, and b+c=1; and a surface treatment layer which is disposed on the core and includes an amorphous oxide containing a lithium oxide, a boron oxide, and an aluminum oxide, wherein the lithium oxide, the boron oxide, and the aluminum oxide are chemically bonded to one another and wherein the lithium oxide is Li 2 O, the boron oxide is B 2 O 3, and the aluminum oxide is Al 2 O 3; and in the amorphous oxide in the surface treatment layer, an amount of the aluminum oxide is greater than an amount of the boron oxide, and an amount of a lithium by-product comprising at least one of LiOH or Li 2 CO 3 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.