Since the 1970s, Laves intermetallics (AB₂) have been widely used in hydrogen storage technology (e.g., nickel–metal hydride batteries) due to the abundant interstitial sites and moderate metal–hydrogen bond strength (E_M–H). They, however, have been rarely used in the hydrogen evolution reaction (HER) because of the same reason (i.e. moderate E_M–H), which results in poor HER efficiency. In this study, by applying lanthanide contraction and ligand effect, we have successfully lowered the E_M–H and substantially boosted the HER activity of Laves intermetallics (RECo₂ and RERu_0.5Co_1.5 (RE = Pr, Tb, Y and Er)) to outperform those of commercial Pt/C catalyst. Hydrogen overpotential decreases from ErCo₂ (η₁₀ = 169 mV) to PrCo₂ (η₁₀ = 113 mV) and then to PrRu_0.5Co_1.5 (η₁₀ = 29 mV). The expansion of lattice constants for PrCo₂ may alleviate the obstacle of H atom diffusing through interstitial sites, while the inclusion of Ru element can raise the antibonding population of Co–Co/Ru bonds, which consequently lowers E_M–H and thus elevates HER activity according to the Sabatier principle. This outcome indicates that the manipulation of the crystal structure and electronic structure factor is an efficient strategy to boost the HER activity of Laves intermetallics.