We have developed a facile approach to engineering ratiometric sensors nano-thermosensors (NTSs) with a core/shell structure. A layer of “smart” thermosensitive poly(N-vinylcaprolactam) doped with quantum dots (QDs) with the emission wavelength λem shell makes it possible to obtain fluorescence emission depending on temperature. QDs with a different emission wavelength λem core incorporated in the NTS core emit a reference signal which allows ratiometric temperature measurements to be performed within a nanoscale volume. It has been shown that an NTS has a high temperature sensitivity if long-wavelength QDs are located in the core of the sensor, whereas the temperature-sensitive shell contains short-wavelength QDs (λem core > λem shell). An optimally selected thermosensitive polymer has allowed us to design NTSs operating in the physiological temperature range.