Organometallic nanosheets are a versatile platform for design of efficient electrocatalyst materials due to their high surface area and uniform dispersion of metal active sites. In this paper, we systematically investigate the electrocatalytic performance of the first transition metal series TM3–C12S12 monolayers on CO2 using spin‐polarized density functional theory. The calculations show that TM3–C12S12 exhibits excellent catalytic activity and selectivity in the catalytic reduction in CO2. The main reduction products of Sc, Ti, and Cr are CH4. V, Mn, Fe and Zn mainly produce HCOOH, and Co produces HCHO, while CO is the main product for Ni and Cu. For Sc, Ti, and Cr, the overpotentials are >0.7 V, while for V, Mn, Fe, Co, Ni, Cu, Zn, the overpotentials are very low and range from 0.27 to 0.47 V. Therefore, our results indicate that many of the TM3–C12S12 monolayers are expected to be excellent and efficient CO2 reduction catalysts.