Unlike the typical low-temperature (<150 °C) continental geothermal systems usually characterized by high N₂, CH₄ and CO₂ concentrations but a trace H₂ concentration, the sandstone-dominated Jimo hot spring on China's eastern coast exhibits: (1) abnormally high H₂ concentrations (2.4–12.5 vol%) and H₂/CH₄ (up to 46.5); (2) depleted δD-H₂ (−709 to −822‰), comparable to the Kansas hot springs near the Mid-Continent rift system with the most depleted δD-H₂ (−740 to −836‰) recorded in nature; and (3) dramatic gas concentration and isotope ratio variations within an area of 0.2 km². Gas chemistry and H-C-He-Ne isotope ratios are studied with reference to published H₂ isotope data from various systems. The origin of the gas is most likely attributed to: (a) allochthonous abiotic H₂ generated by the reduction of water and oxidation of Fe^II-rich pyroxene and olivine (serpentinization) in the basalt located 2 km away under near-surface conditions and migration to the deep sandstone reservoir; (b) primary thermogenic CH₄ produced in the sandstone; (c) mixing with a considerable amount of microbial H₂ from shallow fresh and marine sediments; and (d) biotic CH₄ with typical abiotic signatures resulting from isotope exchanges with fluids high in H₂/CH₄ and CO₂/CH₄ ratios. Allochthonous abiotic H₂ in a sandstone-dominated continental geothermal system and massive microbial fermentation-based H₂ production in shallow fresh and residual marine sediments with insignificant but differential consumption activity are highlighted. The published hydrogen isotope ratios for H₂ produced under various natural geological environmental and experimental conditions have been collected systematically to provide a fundamental framework and an initial tool for restricting the dominant origin of H₂.