The electrocatalytic CO₂ reduction reaction (eCO₂RR) has been gaining increasing attention owing to its potential to contribute to sustainability in our society, although enhanced catalytic performance is a prerequisite for its implementation. Herein, Cu electrocatalysts modified with sulfur proved to selectively produce formate via aqueous eCO₂RR and thus to unexpectedly prevent the mechanistic fingerprint of Cu (i.e., the CO path). Initially, sulfur-modified copper catalysts (Cu–S) were prepared by the in situ reductive reconstruction of nano CuS precursors, revealing a positive correlation between particle size and selectivity toward formate. Subsequent studies over targeted submicron Cu–S particles with varying sulfur content demonstrated their evolution under reaction conditions, attaining a similar surface state comprising metallic Cu and sulfide phases, irrespective of the initial structure of the materials. In accordance, the initial sulfur content showed only a very limited influence on the catalytic performance, which remained at approximately 80% Faradaic efficiency toward formate at −0.8 V vs RHE, outperforming all cost-effective, earth-abundant, and nontoxic electrocatalysts reported to date for the production of formate via the aqueous eCO₂RR.