Developing a noble-metal-free robust and effective electrocatalyst with enhanced active sites for overall water splitting is of great significance yet challenging. Here we demonstrate a bifunctional ultrathin Ni₂P nanosheet electrocatalyst through a microwave-assisted method and phosphorization process. The as-prepared Ni₂P nanosheets present low overpotentials (η₁₀) of 96 and 255 mV for the hydrogen evolution reaction (HER) and oxygen evolution (OER) at a current density of 10 mA cm⁻² in a basic medium. Remarkably, the overall water splitting electrolyzer exhibits a low cell voltage of 1.47 V to achieve a current density of 10 mA cm⁻² in 1 M KOH with outstanding stability. Our facile and unique synthetic protocol and highly active material will open the avenue for further application of water splitting devices.