A fuel cell electrode catalyst includes: catalyst metal particles containing at least one of platinum or a platinum alloy; and support particles supporting the catalyst metal particles. The crystallite size 2r obtained from an X-ray diffraction image of the catalyst metal particles is 3.8 nm or less, where r represents a crystallite radius of the catalyst metal particles obtained from the X-ray diffraction image. The amount of CO adsorption Y (mL/g-Pt) on the fuel cell electrode catalyst satisfies Y≤40.386/r+1.7586.