Abstract: The development of efficient non-precious electrocatalysts for the hydrogen evolution reaction (HER) is crucial for sustainable energy conversion. While two-dimensional Molybdenum Diselenide (MoSe₂) holds huge promise, its performance is constrained by the inert basal planes and poor electrical conductivity. This work proposes an effective strategy for modulating the electronic structure of MoSe₂ through rare-earth (RE) element doping. Systematic investigation reveals that RE doping induces in-plane lattice expansion and acts as a strong electron donor, significantly increasing the electron density at both Mo and Se sites, with erbium identified as the most effective element. Electrochemical evaluations show a universal volcano-shaped dependence of HER activity on doping concentration, with 3% emerging as the optimal level. The optimized catalyst, 3% Er-MoSe₂, exhibits an outstanding HER performance with a low overpotential of 239 mV at 10 mA cm^-2, a small Tafel slope of 88 mV dec^-1, and excellent stability in alkaline media. This work yields profound insight into the role of RE doping in activating and enhancing 2D materials, thereby offering crucial insights for the design of next-generation electrocatalysts.