In the past decade, luminescence thermometry has emerged as an advanced method of remote temperature sensing with high sensitivity and accuracy. Luminescence thermometers comprise primary and secondary sensors, which provide temperature determination based on fundamental physical laws or premeasured calibration, respectively. Here, we propose single Lu2O3:Er3+ nanoparticles to be used as primary and secondary nanoparticles for ratiometric nanothermometry using thermally coupled levels of erbium ion within the range of 123–373 K. The feasibility of temperature sensing based on both photo- and cathodoluminescence spectra further expands the list of possible applications. The best thermometric performance metrics were obtained for the secondary thermometry: Sr = 1.40% K–1 at 298 K and δT = 0.20 K. The demonstrated results open the way to the development of the luminescence thermometers with improved characteristics, which combine primary and secondary strategies.