Abstract: The Guangdong-Hong Kong-Macao Greater Bay Area (GBA) in South China has abundant geothermal resources. However, the geochemistry of geothermal water in the area remains unclear. Hydrochemical analysis of 27 samples shows that the inland and coastal geothermal waters are mainly of bicarbonate and chloride types, respectively. The GBA geothermal water δ²H and δ¹⁸O ratios are in the range of -30‰ to -48‰ and-5.2‰ to -7.5‰, respectively. The δ¹⁸O and δ²H compositions of inland water samples fall along the Local Meteoric Water Line (LMWL), indicating its local precipitation origin. In contrast, coastal geothermal water is enriched in heavy isotopes, with δ¹⁸O values deviating from the LMWL towards those of seawater, with a strong positive correlation between δ¹⁸O values and chloride concentrations, suggesting that coastal geothermal water is recharged by a mixture of seawater and local precipitation. The combination of empirical chemical geothermometers, modeling of multi-mineral saturation states, anhydrite/chalcedony saturation indices, and silica-enthalpy mixing models suggests a reservoir temperature range of 104-156 °C for the GBA geothermal systems. The contribution of cold groundwater to the sampled inland geothermal water ranges from 52%-84% and the proportion of seawater mixing with coastal thermal water can reach up to 37%. The circulation depths of inland and coastal geothermal water are found to vary from 3300-4800 m and 3200-4200 m, respectively. Hydrochemical and isotopic compositions indicate that mixing with cold groundwater/seawater and water-rock interactions are the predominant factors regulating the geochemistry of geothermal water in the GBA, South China.