The synthesis of highly active, stable and cost-effective electrocatalysts for overall water splitting is critical but challenging. Herein, we synthesized nickel-doped pyrrhotite iron sulfide (Fe_0.95−xNi_xS_1.05) nanosheets via a one-pot synthetic route. When tested for oxygen evolution reaction (OER) performance in alkaline solutions, the as-prepared Fe_0.8Ni_0.15S_1.05 catalyst exhibited superior electrocatalytic performance to commercial IrO₂ with a current density of 10 mA cm⁻² at an overpotential of 228 mV, and a Tafel slope of 53 mV dec⁻¹. The hydrogen evolution reaction (HER) activity was achieved on Fe_0.8Ni_0.15S_1.05 with a Tafel slope of 103 mV per decade, and a current density of 10 mA cm⁻² at an overpotential of 263 mV. With nickel doping, the as-prepared pyrrhotite iron sulfide nanosheets can expose more active sites, and hence act as a highly efficient catalyst for overall water splitting.