Electrochemical reduction of CO2 could mitigate environmental problems originating from CO2 emission. Although grain boundaries (GBs) have been tailored to tune binding energies of reaction intermediates and consequently accelerate the CO2 reduction reaction (CO2RR), it is challenging to exclusively clarify the correlation between GBs and enhanced reactivity in nanostructured materials with small dimension (<10 nm). Now, sub‐2 nm SnO2 quantum wires (QWs) composed of individual quantum dots (QDs) and numerous GBs on the surface were synthesized and examined for CO2RR toward HCOOH formation. In contrast to SnO2 nanoparticles (NPs) with a larger electrochemically active surface area (ECSA), the ultrathin SnO2 QWs with exposed GBs show enhanced current density (j), an improved Faradaic efficiency (FE) of over 80 % for HCOOH and ca. 90 % for C1 products as well as energy efficiency (EE) of over 50 % in a wide potential window; maximum values of FE (87.3 %) and EE (52.7 %) are achieved. Ultrathin sub‐2 nm SnO2 quantum wires (QWs) composed of individual quantum dots with grain boundaries (GBs) being terminated on the surface were synthesized and examined for CO2 electrochemical reduction toward HCOOH formation. The ultrathin SnO2 QWs deliver higher current densities and remarkably higher Faraday efficiencies of HCOOH in a wide potential window as compared to SnO2 nanoparticles without exposed GBs.