: Preclinical studies on radiofrequency (RF) cardiac ablation (RFCA) use very small temperature sensors in specific positions in the tissue subjected to RF heating. Despite the sensors’ small size, the proximity to the ablation electrode and the extremely high thermal gradient around the electrode means that the presence of the temperature sensors could distort the temperatures recorded. Our objective was to assess the thermal impact of intra-tissue temperature sensors during RFCA.: 3D RFCA models were built including different temperature sensors based on fiber optics and T-type thermocouples. Constant power ablation was simulated for 10 s.: The results showed that the disturbance caused by the presence of the T-type thermocouples was considerably greater (one order of magnitude) than that caused by the optical fibers. The closer the sensor was to the ablation electrode, the greater the greater the disturbance was and the more it increased with time in sensors more than 3 mm deep. The fiber optic measurements always slightly underestimated (<0.2 °C) the tissue temperature that would exist without the sensors, while the disturbance caused by the T-type thermocouples did not always result in underestimation but depended on the depth of the sensors parallel to the catheter.: The presence of thermocouples inserted into the tissue close to the RF ablation electrode involves a disturbance that could affect the measured temperature value, although it does not substantially alter the shape and size of the thermal lesion. Optical fibers cause much less disturbance, possibly due to the absence of internal metal parts that favor heat conduction.