Photocatalytic overall water splitting is an interesting research topic in the field of energy and outer space exploration. Here, we designed a P/MoS2 composite photocatalyst with self-water-absorption performance by compositing MoS2 nanosheets and polycrystalline black phosphorus nanosheets. The composite photocatalyst produced a sufficient amount of hydrogen in the absence of any precious metal. This also demonstrated photocatalytic overall water-splitting ability in the absence of any sacrificial agent. More interestingly, it effectively absorbed water and demonstrated good overall water-splitting performance under the simulated Mars conditions; the average hydrogen production rate was 17.68 ​μmol ​h−1 ​g−1, and the oxygen production rate was 7.61 ​μmol ​h−1 ​g−1 over a period of 70 ​h. A comprehensive investigation of the ability of the composite photocatalyst to absorb water and produce hydrogen was performed. Here, we designed a P/MoS2 composite photocatalyst with self-water-absorption performance. It demonstrated good water absorption and overall water-splitting properties under simulated Mars conditions. Display omitted •We designed the first P/MoS2 composite photocatalyst with self-water-absorption performance.•The composite photocatalyst effectively produced hydrogen and absorbed water in the absence of any precious metals.•The ability of the photocatalyst to absorb water and perform overall water splitting was confirmed.