Despite extensive studies on CO2 electroreduction, the specific roles of lattice oxygen atoms and hydroxyl groups in the catalytic process are not yet well established. A critical challenge remains in determining whether these lattice oxygen species actively participate in the reaction pathway. Here, we report a lattice oxygen-mediated CO2RR using Sn3O2(OH)2 as a model catalyst for formate electrosynthesis. Employing operando 17O nuclear magnetic resonance (NMR) spectroscopy, we provide direct experimental evidence that formate products (HC17OO– or HC17O17O–) on isotopically labeled Sn317O2(17OH)2 originate from lattice 17O atoms via the lattice oxygen mechanism (LOM). Complementary density functional theory calculations further support the feasibility of a LOM route, which is fundamentally different from the conventional adsorbate evolution mechanism in the CO2RR. The comprehensive operando XAFS analysis and XRD characterizations reveal that the Sn3O2(OH)2 structure is stabilized by chelation with C2O42– ligands. These findings provide new mechanistic insights and design principles for developing highly active and stable oxide catalysts for CO2RR via lattice oxygen participation.