Temperature measurements ranging from few degrees to few hundreds degrees Kelvin are of large interest in the fields of nanomedicine and nanotechnology. Here, we report a new ratiometric luminescent thermometer using thermally excited state absorption of the Eu3+ ion. The thermometer is based on the simple Eu3+ energy level structure and can operate between 180 and 323 K with a relative sensitivity ranging from 0.7 to 1.7 % K−1. The thermometric parameter is defined as the ratio between the emission intensities of the 5D0 → 7F4 transition when the 5D0 emitting level is excited through the 7F2 (physiological range) or the 7F1 (down to 180 K) level. Nano and microcrystals of Y2O3:Eu3+ were chosen as a proof of concept of the operational principles in which both excitation and detection are within the first biological transparent window. A novel and of paramount importance aspect is that the calibration factor can be calculated from the Eu3+ emission spectrum avoiding new calibration procedures whenever the thermometer operates in different media.