Carbonate aquifers in foreland tectonic settings can host important thermal springs although located inareas commonly not characterized by regional high heat flow values. In these cases, when thermal springsare located close or along the coastlines the subaerial and/or submarine thermal springs constitute theoutflow of marine groundwater, flowing through localized fractures and karsitic rock-volumes. This isthe case of springs occurring along the south-easternmost portion of the Apulia region (Southern Italy)where few sulphurous and warm waters (22-33oC) outflow in partially submerged caves located alongthe shoreline, thus supplying the historical spas of Santa Cesarea Terme. Here, with the aim to define theorigin of the thermal fluids and their deep path, we carried out the geo-structural survey of the area anddetailed hydrogeological and geochemical analyses of the thermal spring fluids. In particular, the isotopes18O, D,13C in DIC,34Ssulphate,34Ssulphide,3He/4He ratio and13C in CO2were used to define the origin ofthe thermal water and the recharge mechanism of the geothermal system while the isotopes3H and14Cwere determined for estimating the age of the thermal waters, resulting in older than roughly twentythousands years BP. The results indicate that the thermal springs are fed by marine water, having reachedSanta Cesarea Terme through a localized fracture network. This affects the evaporitic and carbonatic rocksthat characterize the substratum of the Adriatic Sea in the offshore.