A thermometer is described which employs luminescence emitting molecular sensors to measure temperatures in the cryogenic range below 20 K. Upon photoexcitation, the molecular sensor emits luminescence from an excited spin state having two optically resolved sublevels. The sublevels are in thermal equilibrium due to rapid spin‐lattice relaxation and thus act as a two‐level radiator whose relative luminescence intensities vary with temperature according to the Boltzmann population equation. A device is described which employs photon‐counting techniques to measure the intensity ratio of the luminescence originating from these two levels and then uses this ratio to calculate the temperature of the sensor and thereby of its surroundings. The device employs remote fiber‐optic sensing and is independent of fluctuations in the excitation source or in the detection system efficiency since it monitors the intensity ratio of two emission bands. Since this temperature measurement technique is based on photophysical and thermodynamic properties of a well‐defined system, the molecular sensors employed need not be individually calibrated, only the response curve of the device needs to be determined.