MoSe 2 is a 2D layered transition metal dichalcogenide that has attracted much attention because its properties may be easily altered by both morphology control and doping by substitutional transition metals. Here, the study of Ru-doped MoSe 2 nanoflowers is presented, and the effect of Ru doping on their optical, electronic, and catalytic properties is presented. A significant enhancement in their catalytic properties toward the hydrogen evolution reaction (HER) is evident, showing an overpotential as low as 143 mV (at 10 mA cm -2 ) for samples by substituting 11.4% of the Mo with Ru. In order to gain understanding of the dopants' interaction with the host and the nature of the atomic-scale substrate for the catalytic reaction, density functional theory (DFT) calculations are employed to trace the modulation of the density of states (DOS) near the Fermi level and to model possible dopant sites. The Ru dopants have two additional d electrons and a high DOS near the Fermi level. The optical absorption spectra were significantly affected by Ru doping, and the optical band gap of MoSe 2 increased due to the Burstein-Moss effect. The increased charge carrier density enhances the conductance of the samples, but the most significant change is the reduction in the charge transfer resistance during the HER upon doping. © 2018 American Chemical Society.