Developing high-efficiency and stable catalysts from earth-abundant elements for the hydrogen evolution reaction (HER) is essential for renewable energy conversion. In this work, Se-promoted molybdenum sulfide nanosheet arrays supported on flexible carbon cloth (Se–MoS₂/CC) have been successfully synthesized and explored for the first time as a 3D hydrogen evolution cathode. Without any active process, the Se–MoS₂/CC electrode exhibits greatly enhanced activity with a smaller Tafel slope (63 mV dec⁻¹) and a higher exchange current density (0.16 mA cm⁻²) than the pristine MoS₂/CC one (79 mV dec⁻¹ and 0.1 mA cm⁻²). The overpotentials needed to attain the current densities of 10 and 100 mA cm⁻² are merely 127 and 218 mV, respectively. In addition, Se–MoS₂/CC maintains its high electrocatalytic activity for at least 25 h in acidic media.