Monolithic Sn-based catalysts with suitable electronic states and fully exposed active sites are highly desirable to promote the large-scale application of electrochemical conversion of CO₂ into formate. Herein, we highlighted a defect engineering strategy to design and synthesize carbon foam supported oxygen vacancy-rich SnO_x nanosheets as a promising monolithic electrode for electrocatalytic reduction of CO₂ to formate with high activity and selectivity. The obtained monolithic catalyst shows enhanced performance with high formate selectivity (86%) and high formate partial current density (30 mA cm⁻²). The superior performance is attributed to the rich oxygen vacancies confined in SnO_x nanosheets and the three-dimensional interconnected macroporous carbon foam substrate, which causes synergetic effects of higher exposed active sites, faster electron/mass transport, and optimized CO₂ adsorption/activation.