Hierarchical transition-metal dichalcogenide nanosheets are constructed through a versatile strategy, where the thermal polymerization of melamine and subsequent decomposition of carbon nitride successively guide the horizontal and vertical growths of transition-metal chalcogenides. Abundant edges and high surface areas endow the hierarchical MoS2 and WS2 nanosheets with excellent electrocatalytic performance for hydrogen evolution, including low onset potentials and high current densities.