Photocatalysts based on semiconducting conjugated polymers have shown great potential in solar-driven water splitting. The synthetic conjugated polymers offer great versability to design photocatalyts with suitable electronic structures for photocatalytic reactions. So far much efforts have been devoted to developing semiconducting conjugated polymers for photocatalytic hydrogen evolution. However, conjugated polymers that are able to efficiently split pure water under visible light (>400 nm) via a four-electron pathway have been proved to be challenging. We show that 1,3-diyne-linked conjugated polymer nanosheets obtained by oxidative coupling of terminal alkynes such as 1,3,5-tris-(4-ethynylphenyl)-benzene (TEPB) and 1,3,5-triethynylbenzene (TEB) are possessing suitable band structures for photocatalytic overall water splitting and can act as highly efficient photocatalysts for splitting pure water (pH~7) into stoichiometric amounts of H₂ and O₂ under visible light irradiation. We also reveal that these conjugated polymers are indeed able to split pure water via first-principles calculations. Using in situ techniques, we could further elucidate the molecular intermediates that are formed during the photocatalytic process, providing strong evidence that the water splitting reaction could occur on the surface of polymer photocatalysts. We believe that our study could provide new insights in design and synthesis of semiconductors that are able to catalyze overall water splitting at neutral pH with sunlight as the only energy input.