Metallic transition metal dichalcogenides (TMDs)(1-8) are good catalysts for the hydrogen evolution reaction (HER). The overpotential and Tafel slope values of metallic phases and edges(9) of two-dimensional (2D) TMDs approach those of Pt. However, the overall current density of 2D TMD catalysts remains orders of magnitude lower (similar to 10-100 mAcm(-2)) than industrial Pt and Ir electrolysers (>1,000 mAcm(-2))(10,11). Here, we report the synthesis of the metallic 2H phase of niobium disulfide with additional niobium (2H Nb1+xS2, where x is similar to 0.35)(12) as a HER catalyst with current densities of >5,000 mA cm(-2) at similar to 420 mV versus a reversible hydrogen electrode. We find the exchange current density at 0 V for 2H Nb1.35S2 to be similar to 0.8mAcm(-2), corresponding to a turnover frequency of -0.2 s-1. We demonstrate an electrolyser based on a 2H Nb1+xS2 cathode that can generate current densities of 1,000mAcm(-2). Our theoretical results reveal that 2H Nb1+xS2 with Nb-terminated surface has free energy for hydrogen adsorption that is close to thermoneutral, facilitating HER. Therefore, 2H Nb1+xS2 could be a viable catalyst for practical electrolysers.