The sluggish reaction kinetics of the hydrogen evolution reaction (HER) in alkaline media is a great obstacle to alkaline water electrolysis, and it remains a great challenge to develop precious metal-free efficient catalysts for the alkaline HER. Transition metal dichalcogenides (TMDs), in particular MoS2 and MoSe2, are promising catalysts for the HER in acidic media, but they exhibit much inferior catalytic activity for the alkaline HER owing to the slow water dissociation process. In this work, we, for the first time, demonstrate that TMD heterostructures with abundant edge sites deliver substantially accelerated alkaline HER kinetics, which is in great part due to the enhanced water adsorption/dissociation capability. As a proof of concept, MoS2/MoSe2 heterostructures with ultrasmall MoS2 nanoclusters anchored on MoSe2 nanosheets are synthesized via a solution-phase process and are investigated as alkaline HER catalysts in detail. MoSe2 nanosheets serve as excellent substrates to hinder the agglomeration of MoS2 nanoclusters, resulting in abundant edge sites. Benefiting from the decent water adsorption/dissociation capability of the edge sites, the optimal MoS2/MoSe2 heterostructure shows exceptional catalytic activity in 1 M KOH with an overpotential of 235 mV at 10 mA cm-2 and a Tafel slope of 96 mV dec-1, which is substantially improved as compared with the individual MoSe2 (330 mV, 135 mV dec-1) and MoS2 (400 mV, 157 mV dec-1). The success of this catalyst design strategy for enhancing alkaline HER kinetics is also demonstrated in MoSe2/MoSe2 and MoS2/MoS2 heterostructures. The results suggest that engineering additional edge sites that have a strong affinity for H2O is critical for TMDs towards enhanced alkaline HER activity, and also open new avenues in the design of precious metal-free efficient catalysts for the alkaline HER.