Surface roughness in copper catalysts has emerged as a critical factor for enhancing the C2+ product selectivity in electrochemical CO2 reduction (eCO2RR), but the underlying mechanistic origins remain unclear. Here, we employ simulated sputtering and annealing to generate copper surfaces with varying roughness factors. Atomic-scale site-analysis of our model surfaces reveals that roughness increases the density of 4-fold and mixed-coordination sites, promoting CO adsorption and subsequent C–C coupling pathways. Selectivity maps overlaid with site distributions demonstrate that increased surface roughness enhances the fraction of active sites favoring C2+ products, aligning with the experimental trends. Our findings highlight how roughness-induced atomic structural modulation promotes nanoenvironments with mixed high and low coordination 4-fold sites able to sustain C–C coupling, explaining observed selectivity enhancements. This work provides new insights into roughness-dependent site reactivity and offers strategies for rational catalyst design to optimize multicarbon product formation in eCO2RR.