In this communication, nickel diselenide (NiSe2) nanoparticles are synthesized by a facile and low-cost hydrothermal method. The synthesis method can be extended to other metal diselenides as well. The electrode made of NiSe2 exhibits superior electrocatalytic activity in the hydrogen evolution reaction (HER). A low Tafel slope of 31.1 mV per decade is achieved for NiSe2, which is comparable to that of platinum (∼30 mV per decade). Moreover, the catalytic activity of NiSe2 is very stable and no obvious degradation is found even after 1000 cyclic voltammetric sweeps.