A method of preparing a positive electrode active material precursor represented by Ni x Co y Mn z M1 w 2 wherein M1 includes at least one selected from the group consisting of B, Al, Zr, Y, Mo, Cr, V, W, Ta, and Nb, 0.8≤x≤1.0, 0≤y≤0.2, 0≤z≤0.2, 0≤w≤0.15, and x+y+z+w=1, the method comprising: providing a transition metal-containing solution including nickel, cobalt, and manganese; and introducing the transition metal-containing solution into a reactor, adding a basic aqueous solution and an ammonium cation-containing complex-forming agent, and performing a co-precipitation reaction to prepare a transition metal hydroxide in a form of a secondary particle, wherein the secondary particle is formed by agglomerating primary particles, wherein the transition metal-containing solution includes nickel at 80 mol% or more with respect to the total number of moles of transition metals, wherein the ammonium cation-containing complex-forming agent is added wherein a ratio of the number of moles of ammonia included in the ammonium cation-containing complex-forming agent to the total number of moles of transition metals included in the transition metal-containing solution is 0.3 to 0.8, wherein a temperature of the co-precipitation reaction is 40 °C or more and less than 60 °C, and wherein a pH of the co-precipitation reaction is 10 to 12, wherein the co-precipitation reaction is performed under conditions satisfying the following Expression 1 and Expression 2: 28 ≤ 1.8 × X + 0.2 × Y + 20 × Z < 30 in Expression 1, X is a ratio of a number of moles of ammonia included in the ammonium cation-containing complex-forming agent to a total number of moles of transition metals included in the transition metal-containing solution, Y is a value of a temperature (°C) of the co-precipitation reaction, and Z is a ratio of a number of moles of nickel to the total number of moles of transition metals included in the transition metal-containing solution; 12 ≤ A + 0.02 × Y ≤ 13 in Expression 2, A represents a pH of the co-precipitation reaction, and Y represents a value of a temperature (°C) of the co-precipitation reaction. providing a transition metal-containing solution including nickel, cobalt, and manganese; and introducing the transition metal-containing solution into a reactor, adding a basic aqueous solution and an ammonium cation-containing complex-forming agent, and performing a co-precipitation reaction to prepare a transition metal hydroxide in a form of a secondary particle, wherein the secondary particle is formed by agglomerating primary particles, wherein the transition metal-containing solution includes nickel at 80 mol% or more with respect to the total number of moles of transition metals, wherein the ammonium cation-containing complex-forming agent is added wherein a ratio of the number of moles of ammonia included in the ammonium cation-containing complex-forming agent to the total number of moles of transition metals included in the transition metal-containing solution is 0.3 to 0.8, wherein a temperature of the co-precipitation reaction is 40 °C or more and less than 60 °C, and wherein a pH of the co-precipitation reaction is 10 to 12, wherein the co-precipitation reaction is performed under conditions satisfying the following Expression 1 and Expression 2: 28 ≤ 1.8 × X + 0.2 × Y + 20 × Z < 30 in Expression 1, X is a ratio of a number of moles of ammonia included in the ammonium cation-containing complex-forming agent to a total number of moles of transition metals included in the transition metal-containing solution, Y is a value of a temperature (°C) of the co-precipitation reaction, and Z is a ratio of a number of moles of nickel to the total number of moles of transition metals included in the transition metal-containing solution; 12 ≤ A + 0.02 × Y ≤ 13 in Expression 2, A represents a pH of the co-precipitation reaction, and Y represents a value of a temperature (°C) of the co-precipitation reaction.