The temperature dependency of dielectric properties of tissue mimicking materials and tissues is a crucial factor for non-invasive temperature monitoring in microwave thermotherapy. This contribution deals with temperature dependent dielectric spectroscopy measurements by means of ultrawideband sensing. Beside the standard three term calibration, a temperature dependent calibration approach is supposed. Both methods are validated by measuring the dielectric properties of distilled water in the frequency range of 0.5 GHz up to 7 GHz and in the temperature range of 26 °C up to 50 °C. Compared with the Ellison model as reference, the temperature dependent calibration approach shows better results than the standard calibration at room temperature. Furthermore, the temperature dependent dielectric properties of a tumor mimicking agar phantom in the same frequency and temperature range are presented.