Low-temperature electricity-driven water splitting is an established technology for hydrogen production, yet only few materials are able to catalyze hydrogen and oxygen evolution reactions in the same medium. Herein, ultrathin CoP nanosheets (CoP NS) as durable bifunctional catalysts for electrochemical water splitting are reported. The OER and HER activity for CoP NS/C reaching 10 mA cm⁻² needs an overpotential of only 0.277 V and 0.111 V in a basic solution. What's more, when integrated into a practical anion exchange membrane water electrolysis cell using CoP NS as both anode and cathode catalysts, a current density of 335 mA cm⁻² at 1.8 V is achieved, which is rather competitive to the state-of-the-art Pt/IrO₂ catalyst. This work would open a new avenue to explore the use of transition metal phosphides as green and attractive bifunctional catalysts toward mass production of hydrogen fuel for applications.