In this work we present a fabrication method for developing thermal sensors on flexible organic substrates. The constructed devices consist of Pt resistors which are directly integrated to the copper tracks of a flexible copper-clad laminate. They reside on top of a 12 $\mu $m thick SU-8 planarization layer, while a sacrificial layer utilized by the negative photoresist ma-N was used in order to define the resistor pattern. The resistors can act as both heating and temperature sensing elements, while due to small thickness and the low thermal conductivity of the Kapton substrate, a very effective thermal isolation is achieved. The minimum radius of curvature of the fabricated devices was found to be 5 mm. As the device is in direct communication to the macrowolrd, the need for wire bonding is eliminated, while the final surface of the produced sensor is relatively planar. The overall process is simple and cost-effective with minimal requirements in fabrication time. The potential application field of the presented devices is considered quite extensive as they can be directly expanded into flexible sensors able to measure quantities such as fluid flow rate, displacement or vacuum.