A positive electrode active material comprising a lithium composite oxide containing at least nickel and titanium, wherein: the lithium composite oxide is secondary particles in which a plurality of primary particles are agglomerated, and the content (mol%) of titanium calculated from the line sum spectrum of titanium obtained through EDX line scanning for the short axis direction of the primary particle selected from the cross-sectional TEM image of the lithium composite oxide satisfies the following Equation 1: 1.4 ≤ a1 + a2 / 2 / b1 ≤ 3.25 wherein, when the short axis diameter of the primary particle selected from the cross-sectional TEM image is r, a1 is the average content (mol%) of titanium measured based on all metal elements excluding lithium in the region of 0 to 0.05r from the start point of the line sum spectrum, a2 is the average content (mol%) of titanium measured based on all metal elements excluding lithium in the region of 0.95r to r from the start point of the line sum spectrum, and b1 is the average content (mol%) of titanium measured based on all metal elements excluding lithium in the region of 0.05r to 0.95r from the start point of the line sum spectrum, wherein titanium in the lithium composite oxide is present in an amount greater than 0.2 mol% and less than 3.3 mol% based on all metal elements excluding lithium, wherein the lithium composite oxide further comprises at least one metal element selected from cobalt, manganese, and aluminum, and wherein the primary particle is represented by the following Chemical Formula 1: [Chemical Formula 1] Li w Ni 1-(x+y+z) Ti x M1 y M2 z O 2 wherein, M1 is at least one selected from Co, Mn, and Al, M2 is at least one selected from P, Sr, Ba, Zr, Co, Mn, Al, W, Ce, Hf, Ta, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu; M1 and M2 are different from each other, 0.5≤w≤1.5, 0.002<x<0.033, 0<y≤0.40, 0≤z≤0.20. the lithium composite oxide is secondary particles in which a plurality of primary particles are agglomerated, and the content (mol%) of titanium calculated from the line sum spectrum of titanium obtained through EDX line scanning for the short axis direction of the primary particle selected from the cross-sectional TEM image of the lithium composite oxide satisfies the following Equation 1: 1.4 ≤ a1 + a2 / 2 / b1 ≤ 3.25 wherein, when the short axis diameter of the primary particle selected from the cross-sectional TEM image is r, a1 is the average content (mol%) of titanium measured based on all metal elements excluding lithium in the region of 0 to 0.05r from the start point of the line sum spectrum, a2 is the average content (mol%) of titanium measured based on all metal elements excluding lithium in the region of 0.95r to r from the start point of the line sum spectrum, and b1 is the average content (mol%) of titanium measured based on all metal elements excluding lithium in the region of 0.05r to 0.95r from the start point of the line sum spectrum, wherein titanium in the lithium composite oxide is present in an amount greater than 0.2 mol% and less than 3.3 mol% based on all metal elements excluding lithium, wherein the lithium composite oxide further comprises at least one metal element selected from cobalt, manganese, and aluminum, and wherein the primary particle is represented by the following Chemical Formula 1: [Chemical Formula 1] Li w Ni 1-(x+y+z) Ti x M1 y M2 z O 2 wherein, M1 is at least one selected from Co, Mn, and Al, M2 is at least one selected from P, Sr, Ba, Zr, Co, Mn, Al, W, Ce, Hf, Ta, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu; M1 and M2 are different from each other, 0.5≤w≤1.5, 0.002<x<0.033, 0<y≤0.40, 0≤z≤0.20. M1 is at least one selected from Co, Mn, and Al, M2 is at least one selected from P, Sr, Ba, Zr, Co, Mn, Al, W, Ce, Hf, Ta, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, Gd, and Cu; M1 and M2 are different from each other, 0.5≤w≤1.5, 0.002<x<0.033, 0<y≤0.40, 0≤z≤0.20.