The renewable-electricity-powered carbon dioxide reduction (eCO(2)R) to value-added fuels and feedstocks like methane (CH4) holds the sustainable and economically viable carbon cycle at meaningful scales. However, this kinetically challenging eight-electron multistep deep-reduction encounters insufficient catalyst design principles to steer complex CO2 reduction pathways. Utilizing atomic copper (Cu) structures with unitary active sites can boost eCO(2)R-to-CH4 selectivity due to the efficient suppression of unwanted C & horbar;C coupling. Herein, we report a sequential ion exchange strategy to fabricate periodic Cu single-atom catalysts within a polymeric carbon nitride (PCN) matrix, where the uniformly dispersed, diagonally coordinated N & horbar;Cu & horbar;N configuration hosts low-valent Cu delta+ centers. Leveraging the periodic N-anchoring sites with delocalized pi-electron conjugation in the PCN matrix, the isolated Cu sites are obtained with an interatomic distance of similar to 4.2 & Aring; under high metal-loading conditions. This engineered spatial configuration effectively inhibits C & horbar;C coupling to avoid subsequent multicarbon product formation. The optimized Cu-1/PCN demonstrates exceptional eCO(2)R-to-CH4 performance, achieving 71.1% CH4 Faradaic efficiency with a high partial current density of 426.6 mA cm(-2) at -1.50 V versus reversible hydrogen electrode, outpacing the state-of-the-art catalysts. This work delves into effective concepts for steering desirable reaction pathways via precisely modulating active site structures at the atomic level to create favorable microenvironments.