ABSTRACT The electrochemical reduction of CO 2 to multicarbon (C 2+ ) products offers a promising pathway to replace fossil fuels in the chemical and transportation sectors. However, achieving high C 2+ selectivity requires precisely engineered structures, which, in turn, necessitate advanced synthetic strategies and in situ characterization. Herein, we leverage microfluidic technologies to rationally design and synthesize Cu 2 O nanoparticles with tunable features under laminar flow conditions, thereby providing a previously inaccessible level of control over catalyst structure. By tuning flow parameters within the microfluidic platform, we precisely regulate the reaction‐diffusion interface, enabling fine control over nanoparticle size, morphology, and defect density. The resulting Cu 2 O nanoparticles exhibit a high defect density and intrinsic nanoporosity, two properties known to enhance C 2+ selectivity during CO 2 electroreduction. In contrast, Cu 2 O nanoparticles synthesized via conventional batch methods under identical stoichiometric conditions exhibit larger pore sizes, lower defect densities, and lower C 2+ selectivity. Using in situ liquid‐phase transmission electron microscopy and operando X‐ray absorption spectroscopy, we further elucidate the evolution of both catalyst systems. Finally, we demonstrate that surface modification with polyaromatic films further promotes C 2+ formation. This work highlights microfluidic synthesis as a powerful platform for designing advanced electrocatalysts with tunable structural features and enhanced CO 2 conversion performance to C 2+ products.