AbstractFast ionic conductors have considerable potential to enable technological development for energy storage and conversion. Hydride (H⁻) ions are a unique species because of their natural abundance, light mass, and large polarizability. Herein, we investigate characteristic H⁻ conduction, i.e., fast ionic conduction controlled by a pre-exponential factor. Oxygen-doped LaH₃ (LaH_3−2xO_x) has an optimum ionic conductivity of 2.6 × 10⁻² S cm⁻¹, which to the best of our knowledge is the highest H⁻ conductivity reported to date at intermediate temperatures. With increasing oxygen content, the relatively high activation energy remains unchanged, whereas the pre-exponential factor decreases dramatically. This extraordinarily large pre-exponential factor is explained by introducing temperature-dependent enthalpy, derived from H⁻ trapped by lanthanum ions bonded to oxygen ions. Consequently, light mass and large polarizability of H⁻, and the framework comprising densely packed H⁻ in LaH_3−2xO_x are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H⁻ conduction.