Accurate, noninvasive, and self-referenced temperature measurements at the sub-micrometer scale are of great interest, prompted by the ever-growing demands in the fields of nanotechnology and nanomedicine. The thermal dependence of the phosphor's luminescence provides high detection sensitivity and spatial resolution with short acquisition times in, e.g., biological fluids, strong electromagnetic fields, and fast-moving objects. Here, it is shown that nanoparticles of [(Tb-0.914 Eu-0.086)(2)(PDA)(3)(H2O)]center dot 2H(2)O (PDA = 1,4-phenylenediacetic acid), the first lanthanide-organic framework prepared by the spray-drying method, are excellent nanothermometers operating in the solid state in the 10-325 K range (quantum yield of 0.25 at 370 nm, at room temperature). Intriguingly, this system is the most sensitive cryogenic nanothermometer reported so far, combining high sensitivity (up to 5.96 +/- 0.04% K-1 at 25 K), reproducibility (in excess of 99%), and low-temperature uncertainty (0.02 K at 25 K).