Magnesium is usually a major constituent in natural water but very likely to be depleted in high-temperature geothermal waters hosted by felsic-rocks. In the presented study, the Yunnan-Sichuan-Tibet Geothermal Province (YST) in China was selected as the study area for comprehensively investigating the geochemistry of magnesium in geothermal waters with a wide variety of pH, temperature and chemical composition. As expected, the acid geothermal waters have much higher magnesium concentrations than the neutral waters equilibrated with reservoir minerals to a higher degree. The primary alteration minerals controlling the concentrations of magnesium in neutral to alkaline geothermal waters are chlorites and saponites, as indicated by the evidence from Mg/H-2 - SiO2 activity plots, calculation of saturation indices, and simulation of water-minerals reactions. It is noted that the complexation of magnesium with major anions in geothermal water also has a non-negligible effect on its total magnesium concentration because it increases the solubility of important magnesium-bearing minerals. Environmental temperature, solution pH, and salinity are the principal factors affecting the complexation between magnesium and these common anions. This is the first systematic study on the magnesium geochemistry of the hydrothermal systems in the sole high-temperature geothermal region (YST) in mainland China.