A rational design of highly active and robust catalysts based on earth-abundant elements for hydrogen evolution reaction (HER) is essential for future renewable energy applications. Herein, we report the synthesis of a new class of ultrathin metallic CuFeS2 nanosheets (NSs) with abundant exposed high-index {02[combining macron]4} facets. They serve as a robust catalyst for the HER with a lower onset potential of 28.1 mV, an overpotential of only 88.7 mV (at j = 10 mA cm-2) and remarkable long-term stability in 0.5 M H2SO4, which make them the best system among all the reported non-noble metal catalysts. The theoretical calculations reveal that the mechanistic origin for such a high HER activity should be attributed to the excess S2- active sites on the exposed {02[combining macron]4} high-index facets of CuFeS2 NSs, which have a rather favorable Gibbs free energy for atomic hydrogen adsorption. The present work highlights the importance of designing ultrathin metallic chalcopyrite nanosheets with high-index facets in order to increase the number of active sites for boosting the HER performance.