Metallic bismuth (Bi) holds great promise in efficient conversion of carbon dioxide (CO2) into formate, yet the complicated synthetic routes and unobtrusive performance hinder the practical application. Herein, a facile galvanic‐cell deposition method is proposed for the rapid and one‐step synthesis of Bi nanodendrites. Compared to the traditional deposition method, it is found that the special galvanic‐cell configuration can promote the exposure of low‐angle grain boundaries. X‐ray absorption spectroscopy, in situ characterizations and theoretical calculations indicate the electronical structures can be greatly tailored by the grain boundaries, which can facilitate the CO2 adsorption and intermediate formation. Consequently, the grain boundary‐enriched Bi nanodendrites exhibit a high selectivity toward formate with an impressively high production rate of 557.2 µmol h‐1 cm‐2 at −0.94 V versus reversible hydrogen electrode, which outperforms most of the state‐of‐the‐art Bi‐based electrocatalysts with longer synthesis time. This work provides a straightforward method for rapidly fabricating active Bi electrocatalysts, and explicitly reveals the critical effect of grain boundary in Bi nanostructures on CO2 reduction.