Recently, a large number of nanostructured metal-containing materials have been developed for the electrochemical CO₂ reduction reaction (eCO₂RR). However, it remains a challenge to achieve high activity and selectivity with respect to the metal load due to the limited concentration of surface metal atoms. Here, it is reported that the bismuth-based metal–organic framework Bi(1,3,5-tris(4-carboxyphenyl)benzene), herein denoted Bi(btb), works as a precatalyst and undergoes a structural rearrangement at reducing potentials to form highly active and selective catalytic Bi-based nanoparticles dispersed in a porous organic matrix. The structural change is investigated by electron microscopy, X-ray diffraction, total scattering, and spectroscopic techniques. Due to the periodic arrangement of Bi cations in highly porous Bi(btb), the in situ formed Bi nanoparticles are well-dispersed and hence highly exposed for surface catalytic reactions. As a result, high selectivity over a broad potential range in the eCO₂RR toward formate production with a Faradaic efficiency up to 95(3)% is achieved. Moreover, a large current density with respect to the Bi load, i.e., a mass activity, up to 261(13) A g⁻¹ is achieved, thereby outperforming most other nanostructured Bi materials.