A facile, in-situ deposition route to stable iridium oxide (IrOx·nH2O) nanoparticle thin films from [Ir(OH)6]2− solutions is reported. The [Ir(OH)6]2− solution, made by alkaline hydrolysis of [IrCl6]2−, is colorless and stable near neutral pH, and forms blue IrOx·nH2O nanoparticle suspensions once it is adjusted to acidic or basic conditions. IrOx·nH2O nanoparticle thin films are grown anodically on glassy carbon, fluorine-doped tin oxide, and gold electrodes by electrolyzing [Ir(OH)6]2− solutions at +1.0–1.3 V versus Ag/AgCl. The thickness of the IrOx·nH2O films can be controlled by varying the concentration of [Ir(OH)6]2−, the deposition potential, and/or the deposition time. These thin films are stable between pH 1 and 13 and have the lowest overpotential (η) for the oxygen evolution reaction (OER) of any yet reported. Near neutral pH, the Tafel slope for the OER at a IrOx·nH2O film/Au rotating disk electrode was 37–39 mV per decade. The exchange current density for the OER was 4–8 × 10−10 A cm−2 at a 4 mC cm−2 coverage of electroactive Ir.