An accurate determination of body core temperature is crucial during surgery in order to avoid and treat hypothermia, which is associated with poor outcome. In a prospective observational study, we evaluated the suitability of the Tcore™ device (Drägerwerk AG & Co. KGaA, Lübeck, Germany)—a non-invasive thermometer—to accurately determine core body temperature. In patients undergoing surgery for ovarian cancer, core body temperature (CBT) was determined with the Tcore™ sensor attached to the forehead and compared with blood temperature (T blood ) as measured within the femoro-iliacal artery. Both temperatures were recorded every 10 s and the measurement error was calculated. 57,302 data pairs of CBT and T blood were obtained in 22 patients. In a repeated-measurements version of the Bland and Altman test, a bias of − 0.02 °C and 95% limits of agreement of − 0.48 to 0.44 °C were calculated. In a population analysis, a median absolute error of 0 − 0.1; + 0.1 °C, a bias of 0 − 0.276; 0.271 % and an inaccuracy of 0.276 0.274; 0.354 % was determined. Although the Tcore™ sensor was attached to the frontal skin, it provided an accurate measurement of core body temperature in the investigated intraoperative setting.