The abundant-metal-based polyoxometalate complex [Co-4(H2O)(2)(PW9O34)(2)](10-) is a hydrolytically and oxidatively stable, homogeneous, and efficient molecular catalyst for the visible-light-driven catalytic oxidation of water. Using a sacrificial electron acceptor and photosensitizer, it exhibits a high (30%) photon-to-O-2 yield and a large turnover number (> 220, limited solely by depletion of the sacrificial electron acceptor) at pH 8. The photocatalytic performance of this catalyst is superior to that of the previously reported precious-metal-based polyoxometalate water oxidation catalyst [{Ru4O4(OH)(2)(H2O)(4)}(gamma-SiW10O36)(2)](10-).