Selective electrocatalytic reduction of CO2 to formate has received increasing interest for CO2 conversion and utilization. Yet, the CO2 reduction process still faces major challenges, partly due to the lack of cost-effective, highly active, selective and stable electrocatalysts. Here, we report a mesoporous indium (mp-In) electrocatalyst composed of nanobelts synthesized via a simple solution-based approach for selective CO2 reduction to formate. The mp-In nanocrystals provide enlarged surface areas, abundant surface active sites and edge/low-coordinated sites. Such advantages afford the mp-In with an outstanding electrocatalytic performance for the CO2-to-formate conversion. A high formate selectivity, with a Faradaic efficiency (FE) of >90% was achieved over a potential of −0.95 V to −1.1 V (vs VRHE). The mp-In catalyst showed excellent durability, reflected by the stable formate selectivity and current density over a 24 h reaction period. Density functional theory (DFT) calculations reveal that the stabilization of the intermediate OCHO* on the In-plane surfaces is energetically feasible, further elucidating the origin of its enhanced CO2-to-formate activity and selectivity. This work may offer valuable insights for the facile fabrication of porous hierarchical nanostructures for electrocatalytic and selective reduction of CO2.