Geothermal energy plays a key role in the green energy transition since it represents a low-carbon alternative to traditional fuels, but the high exploration costs and mining risks still hinder its use. Therefore, the reuse of pre-existing subsurface geological data can represent a way to counteract these limiting factors and promote the use of geothermal resources. We reconstructed a 3D geological model of the Guardia Lombardi area (Campania Region, southern Italy) and evaluated its geothermal potential interpreting vintage oil and gas subsurface data (i.e., seismic reflection profiles and well data). The exploitation potential of the geothermal resource and the related costs were also evaluated. The study revealed the presence of a geothermal reservoir with 125 °C at just 2300 m depth and an exploitation potential of about 70 kg/s, employable for residential heating and/or cooling and for electricity production in the nearby Grottaminarda town, with an appreciable economic benefit. These results demonstrate how the reuse of pre-existing subsurface geological data provided by past oil & gas exploration can considerably reduce costs and mining risks associated with geothermal resource exploration, contributing to a faster and considerable reduction in CO2 emissions.