Developing highly active, durable, and cost‐effective electrocatalysts for the oxygen evolution reaction (OER) is of prime importance in proton exchange membrane (PEM) water electrolysis techniques. Herein, a surface lanthanum‐deficient (SLD) iridium oxide as a highly efficient OER electrocatalyst is reported (labeled as La₃IrO₇‐SLD), which is obtained by electrochemical activation, and shows better activity and durability than that of commerically available IrO₂ as well as most of the reported Ir‐based OER electrocatalysts. At a current density of 10 mA cm⁻², the overpotential of La₃IrO₇‐SLD is 296 mV, which is lower than that of IrO₂ (316 mV). Impressively, the increase of potential is less than 50 mV at a voltage–time chronopotentiometry extending for 60 000 s using a glass carbon electrode that is vastly superior to IrO₂. Moreover, the mass activity of the catalyst is approximately five times higher than that of IrO₂ at 1.60 V versus reversible hydrogen electrode. Density functional theory calculations suggest that a lattice oxygen participating mechanism with central Ir atoms serving as active sites (LOM‐Ir) rationalizes the high activity and durability for the La₃IrO₇‐SLD electrocatalyst. The favorable energy level of surface active Ir 5d orbitals relative to coordinated O 2p orbitals makes the La₃IrO₇‐SLD more active.