Crystalline　red　phosphorus　(CRP),　a　newly　emerging　photocatalyst,　shows　promise　for　selective　CH4　production　from　CO2　and　H2O.　Yet,　its　potential　remains　largely　untapped,　particularly　in　enhancing　photocatalytic　performance.　We　address　this　gap　by　incorporating　RuCu　alloy　nanoparticles　into　CRP,　significantly　boosting　its　efficiency　in　converting　CO2　to　CH4.　Advanced　analyses　demonstrate　that　the　synergistic　mechanism　between　the　RuCu　alloy　and　CRP　facilitates　improved　charge　separation　and　expands　the　availability　of　active　sites,　leading　to　an　impressive　CH4　yield　of　19.7　mu　mol　g(-1)　h(-1).　Importantly,　the　inclusion　of　Cu　in　the　alloy　modifies　the　catalytic　reaction　pathways　facilitated　by　Ru,　achieving　an　exceptional　CO2　to　CH4　selectivity　of　96　%.　This　performance　surpasses　that　of　many　existing　elemental-based　and　Ru-containing　photocatalysts.　Our　research　highlights　the　potential　of　functionalized　elemental　photocatalysts　and　the　importance　of　alloy　composition　in　photocatalyst　design　for　sustainable　CO2　conversion.