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. <EOS>