Abstract: Heterojunction catalysts composed of transition metal sulfides exhibited excellent potentials in electrocatalytic water splitting. Herein, we have designed a FeS/Co₃S₄ heterojunction catalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline water/seawater solution. Three-dimensional nanoarrays were grown on nickel foam, and the successful synthesis of heterojunction endowed excellent activity to the catalyst. In alkaline water/seawater solution, the low overpotentials (at current density of 10 mA/cm²) of HER were 120.3 and 135.6 mV and the low overpotentials of OER were 212 and 232 mV, respectively. This work provided an effective method for highly-efficiently electrocatalytic splitting of water via fabrication of heterojunction.