The doping concentration of lanthanide ions is important for manipulating the luminescence properties of upconversion nanoparticles (UCNPs). However, the serious concentration quenching in highly doped UCNPs remains a vital restriction for further enhanced upconversion luminescence (UCL). Herein, we examined the effect of temperature on the concentration quenching of rare-earth UCNPs, an issue that has been overlooked, and we show that it is significant for biomedical or optical applications of UCNPs. In this work, we prepared a series of UCNPs by doping Er³⁺ luminescent centers at different concentrations in a NaLuF₄: Yb³⁺ matrix. At room temperature (298 K), steady-state photoluminescence (PL) spectroscopy showed substantial concentration quenching of the Er³⁺ emission with increasing doping concentrations. However, the concentration quenching effect was no longer effective at lower temperatures. Kinetic curves obtained from time-resolved PL spectroscopy further showed that the concentration quenching dynamics were vitally altered in the cryogenic temperature region, i.e., below 160 K. Our work on the temperature-switchable concentration quenching mechanism may shed light on improving UCL properties, promoting their practical applications.