A positive electrode active material for a rechargeable lithium battery, and a rechargeable lithium battery including the same are provided. The positive electrode active material includes a lithium nickel-based composite oxide. The positive electrode active material is in a form of secondary particles in which a plurality of primary particles are aggregated and at least a portion of the primary particles are radially arranged, and a full width at half maximum (FWHM) of the peak corresponding to the (003) plane in the X-ray diffraction analysis for the positive electrode active material is less than or equal to about 0.125. The lithium nickel-based composite oxide is represented by Chemical Formula 1-1, the substituents of which are the same as described in the detailed description: