Geothermal fields emit remarkable amounts of mercury (Hg) to the environment. To address the source, fate and geochemical cycling of Hg in geothermal fields, we investigated Hg concentrations and isotopic compositions of hot spring water and fumarole gases from Rehai and Dagejia in SW‐China. Elevated Hg concentrations in fumarole gases (10.0–167 ng m 3) and hot spring water (3.44–84.5 ng L 1) were observed, suggesting that both geothermal fields are of environmental concern. The variation in Δ199Hg ( 0.06 to 0.23‰) and Δ200Hg ( 0.09 to 0.19‰) in hot spring water supports Hg likely originating from endogenous volcanic degassing and/or rainwater. Negative and nearly zero Δ199Hg in fumarole gases ( 0.32 to 0.03‰) supports volcanic degassing and background atmosphere origin. The ranges of δ202Hg in fumarole gases ( 0.74 to 0.59‰) and hot spring water ( 1.29 to 0.52‰) could reflect limited fluids boiling in geothermal fields. This study, thus, fills an important knowledge gap regarding Hg global cycling.