MoSe2-reduced graphene oxide (rGO) composite powders with unique structures containing empty nanovoids and excellent hydrogen evolution reaction (HER) performances were prepared using a pilot scale spray-drying process. One-step post-treatment of the spray-dried powders produced the macroporous MoSe2-rGO composite with empty nanovoids through an intermediate MoO3-Se-GO composite. Ultrafine MoSe2 nanocrystals, which consisted of a few layers and were several" nanometers in size, were uniformly dispersed on the surfaces of the MoSe2-rGO composite powders with the optimal rGO contents. The MoSe2-rGO-M composite (20 wt% rGO) had the optimized porous structure with a uniform distribution of MoSe2 nanocrystals and enough the rGO content for a fast electron transport, and thus exhibited the highest HER activity. The MoSe2-rGO-M composite powders exhibited a current density of 10 mA cm(-2) at a small overpotential of 0.21 V, which was lower than that of bare MoSe2 (10 mA cm (-2) at 0.31 V). The Tafel slopes for bare MoSe2 and MoSe2-rGO-M composite powders were 120 and 57 mV dec(-1), respectively. The synergistic effects of rGO sheets and MoSe2 nanocrystals and the unique porous structure resulted in outstanding HER performance with a small Tafel slope and overpotential. (C) 2017 Elsevier B.V. All rights reserved.