The Hamamayağı Thermal Spring is located to the south of Samsun, within the North Anatolian Fault Zone. The units exposed in the Hamamayağı Thermal Spring and vicinity are composed of sedimentary rocks intercalated with volcanics ranging from Permian to Pliocene in age. The temperature, pH value, electrical conductivity and total dissolved solids of the spring water are 36oC, 6.9-7.7, 591-598 μS/cm and 485.6-508.5 mg/l, respectively. The aim of this study is to determine of the temperature of the geothermal reservoir that might be used for different purposes. In this study, the Hamamayağı thermal spring, Hamamayağı stream and Kocapınar fountain representing cold groundwater are investigated in terms of their geochemical and isotopic properties. Based on the $delta^ {18}O$, $delta^ 2H,$ and $^ 3H$ isotope data, the Hamamayağı thermal spring is of meteoric origin. The $delta^ {13}$C values for dissolved inorganic carbonate in the thermal waters, in the Kocapınar fountain and in the Hamamayağı Stream range from -1.78 to -1.62‰, -12.18 to -8.25‰ and -10.10 to -8.31‰ respectively. The carbon in the Hamamayağı thermal spring originates from the dissolution of marine and fresh water carbonates. The d34S content of the sulfate in the thermal waters ranges between -4.2 and -4.1‰. d34S values are related to diagenetic environments which are characterized by reduced sulfur compounds.The reservoir temperature of the Hamamayağı thermal field that is characterized by a low enthalpy geothermal system lies roughly between 61.63- 82.5oC as calculated using a silica geothermometer. In the Hamamayagı geothermal field locted in the North Anatolian Fault Zone, shallow reservoir rocks and an active fault system provide large amounts of groundwater circulation. Therefore, the temperature of geothermal waters, water-rock interaction and related total dissolved solids contents, especially those of boron, lithium and silica which are dissolved at high temperatures, are all low.