Elemental　P　is　considered　a　compelling　option　for　constructing　a　simple,　cost-effective,　and　full-spectrum　responsive　catalytic　system　for　hydrogen　production,　while　its　full　potential　for　overall　water-splitting　reactions　remains　underexplored.　This　study　introduces　a　novel　cobalt　single-atom-assisted　photocatalytic　system　for　efficient　hydrogen　production　via　water　splitting.　Utilizing　an　in　situ　surface　phase　transformation,　amorphous　red　phosphorus　(a-RP)　is　converted　into　crystalline　black　phosphorus　(c-BP),　forming　a　Z-scheme　heterojunction　with　cobalt　single　atoms　(Co1)　uniformly　dispersed　across　the　heterojunction　surface.　Advanced　characterization　techniques　confirm　the　intimate　contact　and　strong　electronic　interactions　between　Co1,　c-BP,　and　a-RP,　significantly　enhancing　photocatalytic　performance.　This　system　achieves　a　high　hydrogen　production　rate　of　approximate　to　2497　mu　mol　g-1　h-1　from　pure　water　splitting　under　visible　light　irradiation　and　a　remarkable　solar-to-hydrogen　(STH)　efficiency　of　0.86%,　outperforming　most　reported　photocatalytic　systems.　This　study　highlights　the　potential　of　single-atom　catalysts　in　enhancing　the　performance　of　photocatalysts.　It　provides　a　new　perspective　on　designing　efficient　and　stable　elemental-based　photocatalytic　systems　for　sustainable　hydrogen　production.