The development of highly efficient electrocatalysts toward the oxygen evolution reaction is imperative for advancing water splitting technology to generate clean hydrogen energy. Herein, a two dimensional (2D) nanosheet ammonium cobalt phosphate hydrate (NH 4 CoPO 4 •H 2 O) catalyst based on the earth-abundant non-noble metal is reported. When used for the challenging alkaline saline water electrolysis, the NH 4 CoPO 4 •H 2 O catalyst with the optimal thickness of 30 nm achieves current densities of 10 and 100 mA cm −2 at the record low overpotentials of 252 and 268 mV, respectively, while maintaining remarkable stability during the alkaline saline water oxidation at room temperature. X-ray absorption fine spectra reveal that the activation of Co (II) ions (in NH 4 CoPO 4 •H 2 O) to Co (III) species constructs the electrocatalytic active sites. The 2D nanosheet morphology of NH 4 CoPO 4 •H 2 O provides a larger active surface area and more surface-exposed active sites, which enable the nanosheet catalyst to facilitate the alkaline freshwater and simulated seawater oxidation with excellent activity. The facile and environmentally-benign H 2 O-mediated synthesis route under mild condition makes NH 4 CoPO 4 •H 2 O catalyst highly feasible for practical manufacturing. In comparison with noble metals, this novel electrocatalyst offers a cost-effective alternative for economic saline water oxidation to advance water electrolysis technology.