Current Sn-based materials are ideal catalysts developed to drive the electrochemical conversion of CO₂ to formate, but competing proton reduction to hydrogen is an ever-present drain on catalytic selectivity. Herein, we report a reliable electrochemical exfoliation route, with the assistance of alternating voltage, for large-scale preparation of two-dimensional (2D) ultrathin tin monosulfide nanosheets (SnS NSs), which feature a large lateral size of 1.0 μm with a thickness of ∼5.0 nm. Systematic electrochemical studies demonstrated that the achieved SnS NSs exhibited an outstanding electrocatalytic activity towards the CO₂ reduction reaction (CO₂RR) to the formate product, as evidenced by a considerable faradaic efficiency (F.E.) of 82.1%, a high partial current density of 18.9 mA cm^-2 at -1.1 V, and a low Tafel slope of 180 mV dec^-1. Further, using an electrode prepared from the resulting SnS NSs by the particle transfer method, a remarkably high formate F.E. over 91% was achieved in a wide potential window. Such high performance renders the SnS NSs among the best reported tin sulfide-based CO₂RR electrocatalysts. Theoretical calculations coupled with comprehensive experimental studies demonstrated that the synergistic effect between the ultrathin layered architecture and dominantly exposed (001) plane of SnS NSs accounted for the uniquely efficient catalytic activity for the CO₂RR.