Using clean electricity to convert carbon dioxide (CO2), an earth abundant carbon feedstock, to high-energy fuels is a fascinating energy strategy for a sustainable future. However, high yield of multi-carbon products remains a grand challenge. Herein, we show that trimeric metal clusters anchored on nitrogen doped porous carbon materials possess remarkable activity and selectivity for C2–C3 hydrocarbons and alcohols. By comprehensive ab initio calculations on 3d, 4d and 5d transition metal trimers, we for the first time illuminate their specialty for catalyzing this tough reaction. These spatially confined triatomic metal centers are capable of simultaneous fixation of plural CO2 molecules, provide exclusive reaction channels that sterically and electronically facilitate C–C coupling, and have tunable selectivity by mediating the cluster-substrate interaction. The catalytic mechanism and structure-activity relationship are uncovered for these sub-nano clusters with robust stability. These results provide important knowledge for atomically precise design of novel catalysts for direct conversion of CO2 to high-energy fuels and high-value chemicals.