Supported metal (oxide) clusters, with both rich surface sites and high atom utilization efficiency, have shown improved activity and selectivity for many catalytic reactions over nanoparticle and single atom catalysts. Yet, the role of cluster catalysts has been rarely reported in CO₂ electroreduction reaction (CO₂RR), which is a promising route for converting CO₂ to liquid fuels like formic acid with renewable electricity. Here we develop a bismuth oxide (BiO_n) cluster catalyst for highly efficient CO₂RR to formate. The BiO_n cluster catalyst exhibits excellent activity, selectivity, and stability towards formate production, with a formate Faradaic efficiency of over 90% at a current density up to 500 mA·cm⁻² in an alkaline membrane electrode assembly electrolyzer, corresponding to a mass activity as high as 3,750 A·g_Bi⁻¹. The electrolyzer with the BiO_n cluster catalyst delivers a remarkable formate production rate of 0.56 mmol·min⁻¹ at a high single-pass CO₂ conversion of 44%. Density functional theory calculations indicate that Bi₄O₃ cluster is more favorable for stabilizing the HCOO* intermediate than Bi(001) surface and single site BiC₄ motif, rationalizing the improved formate production over the BiO_n cluster catalyst. This work highlights the great importance of cluster catalysts in activity and selectivity control in electrocatalytic CO₂ conversion.