A positive electrode active material according to the present disclosure includes a lithium composite oxide. The lithium composite oxide is a multiphase mixture including a first phase having a crystal structure belonging to space group C2/m and a second phase having a crystal structure belonging to space group R-3m and includes at least one selected from the group consisting of F, Cl, N, and S. In an XRD pattern of the lithium composite oxide, the integrated intensity ratio I(20°-23°)/I(18°-20°) of a second maximum peak present in a diffraction angle 2θ range of greater than or equal to 20° and less than or equal to 23° to a first maximum peak present in a diffraction angle 2θ range of greater than or equal to 18° and less than or equal to 20° satisfies 0.05≤I(20°-23°)/I(18°-20°)≤0.26.