To overcome the bottleneck of water splitting, the exploration of efficient, selective, and stable water oxidation catalysts (WOCs) is crucial. We report an all-inorganic, oxidatively and hydrolytically stable WOC based on a polyoxometalate [(A--SiW₉O₃₄)₂Co₈(OH)₆(H₂O)₂(CO₃)₃]¹⁶⁻ (Co₈POM). As a cobalt(II)-based cubane water oxidation catalyst, Co₈POM embeds double Co^II₄O₃ cores. The self-assembled catalyst is similar to the oxygen evolving complex (OEC) of photosystem II (PS II). Using [Ru(bpy)₃]²⁺ as a photosensitizer and persulfate as a sacrificial electron acceptor, Co₈POM exhibits excellent water oxidation activity with a turnover number (TON) of 1436, currently the highest among bioinspired catalysts with a cubical core, and a high initial turnover frequency (TOF). Investigation by several spectroscopy, spectrometry, and other techniques confirm that Co₈POM is a stable and efficient catalyst for visible light-driven water oxidation. The results offer a useful insight into the design of water oxidation catalysts.