Electrochemical water splitting is promising for utilizing intermittent renewable energy. The sluggish kinetics of the oxygen evolution reaction (OER), however, is a bottleneck in obtaining high efficiency. Only a few OER electrocatalysts have been developed for the use in acidic media despite the importance of a proton exchange membrane (PEM) water electrolyzer. IrO2 is the only material that is both active and stable for the OER in highly corrosive acidic conditions. Herein, a facile and scalable synthesis of ultrathin IrO2 nanoneedles is reported with a diameter of 2 nm using a modified molten salt method. The activity and durability for the OER are significantly enhanced on the ultrathin IrO2 nanoneedles, compared to conventional nanoparticles. The ultrathin nanoneedles are successfully introduced to a PEM electrolyzer single cell with the enhanced cell performance.