In this study, the hydrogeological, hydrogeochemical and isotopic signatures of the thermal waters in Kusadasi and surroundings will be presented. In the study area, the basement rocks are impermeable Paleozoic mica schists of the Menderes Massif. The Mesozoic marbles are ascribed either to groundwater aquifers or to thermal waters reservoirs. These rocks are covered by the Kusadasi formation consisting of a sequence of claystones, conglomerates and carbonate rocks. This upper Kusadasi formation plays an important role as cap rocks. The existence of active faults and basic volcanic rocks of Pliocene age points to the existence of high heat flows and geothermal gradients. As a youngest geological formation, the alluvium is a very good aquifer. Nowadays, there is the risk of sea water intrusion into the local groundwaters due to the drop of groundwater table caused by groundwater overexploitation in the study area. (C) 2017 The Authors. Published by Elsevier B.V. is an open access article under the CC BY- NC- ND license.