Atomic monolayer islands of Pt, namely, two-dimensional Pt nanoparticles, on a Au(111) electrode have been studied for the first time, focusing on their electrocatalytic activities for oxygen reduction in acid solutions. The Pt islands' electrodes were prepared using the self-assembled technique of thiols together with the replacement of Pt with a Cu monolayer. The states of adsorbed OH and the catalytic activities of oxygen reduction were sensitive to the Pt island size. As island size decreased, a delay in the reduction of surface oxide was observed. However, negligible influence of adsorbed OH on activity for oxygen reduction was observed. Pt islands of sizes ranging from 5 to 10 nm showed higher specific catalytic activities for oxygen reduction. Specific catalytic activities decreased by a factor of 10 with a decrease in island sizes from 5.5 to 3.1 nm. Size effects observed in Pt monolayer islands were discussed in comparison with three-dimensional nanoparticles, to obtain information concerning the size effects of metal nanoparticles.