Copper nanoparticles (Cu NPs) are effective catalysts for the electroreduction of CO2 (ECO2R) to multicarbon products but suffer from insufficient selectivity, aggregation, and deactivation. To address these challenges, we developed an in situ encapsulation strategy that engineers Cu NPs in a metal–organic framework (MOF) host from a simple one-pot hydrothermal synthesis, creating a selective and robust CO2R catalyst. The key design is the introduction of Sn additives during synthesis, which later evolve into single atoms (SAs) that serve a dual function: modulating the growth of Cu NPs from 3.35 to 9 nm and acting as active sites for the conversion of CO2 to CO. The locally generated CO then feeds adjacent Cu NPs, promoting subsequent C–C coupling via a tandem mechanism. The optimal catalyst, with a balanced Cu/Sn ratio, achieves a CO2-to-C2H4 Faradaic efficiency (FE) of 64%. Combined theoretical simulations and in situ infrared spectroscopy further reveal that Sn SAs promote Cu NPs electron transfer, enriching the electron density at active sites. This stabilizes *CO intermediates and reduces the energy barriers for CO2 activation and ensuing C–C coupling steps. This work presents a novel atomic- and nanoscale design strategy for advanced CO2RR catalysts.