Abstract Main‐group element indium (In) is a promising electrocatalyst which triggers CO 2 reduction to formate, while the high overpotential and low Faradaic efficiency (FE) hinder its practical application. Herein, we rationally design a new In single‐atom catalyst containing exclusive isolated In δ+ –N 4 atomic interface sites for CO 2 electroreduction to formate with high efficiency. This catalyst exhibits an extremely large turnover frequency (TOF) up to 12500 h −1 at −0.95 V versus the reversible hydrogen electrode (RHE), with a FE for formate of 96 % and current density of 8.87 mA cm −2 at low potential of −0.65 V versus RHE. Our findings present a feasible strategy for the accurate regulation of main‐group indium catalysts for CO 2 reduction at atomic scale.