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