This paper contains brief geological and hydrogeochemical information regarding hot waters circulating through the Seferihisar geothermal system using previous studies, as well as recorded and up-to-date data. The obtained analytical results were subjected to the computer programme AquaChem3.70 for determining the geothermal waters. The NaCl-type geothermal waters circulating in the system have a temperature range of 44 to 207 °C, with total dissolved solids (TDS) varying from 3229 to 29494.9 mg/l. The conductivity range of the samples is 5690–34400 µS/cm as the pH changes from 6.54 to 8.5. Chemical geothermometer calculations indicate reservoir temperatures varying between 59 and 246.2 °C. The water‒rock interactions led to various mineralogical changes in the area. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray (XRD and SEM–EDX) analyses of the collected rock and clay samples have been interpreted as an integrant to the 2-year field observations. The results of the analyses show that the surface manifestations in the Tuzla and Doğanbey geothermal fields consist of the hydrothermal alteration products induced from the carbonates of sediments, ferromagnesian minerals of greenschist facies and silicates. Additionally, a uranium-oxide mineral, richetite, has been identified. Finally, the apparent geothermal potential of the system has been assessed.