As a 100% atom-economy process, direct oxidation of methane into methanol remains as a grand challenge due to the dilemma between activation of methane and over-oxidation of methanol. Here, we report that water enabled mild oxidation of methane into methanol with >99% selectivity over Au single atoms on black phosphorus (Au/BP) nanosheets under light irradiation. The mass activity of Au/BP nanosheets reached 113.5 μmol g in water pressured with 33 bar of mixed gas (CH:O = 10:1) at 90 °C under light irradiation (1.2 W), while the activation energy was 43.4 kJ mol. Mechanistic studies revealed that water assisted the activation of O to generate reactive hydroxyl groups and •OH radicals under light irradiation. Hydroxyl groups reacted with methane at Au single atoms to form water and CH* species, followed by oxidation of CH* via •OH radicals into methanol. Considering the recycling of water during the whole process, we can also regard water as a catalyst.