Fluorescent probes that are able to directly measure viscosity are attractive candidates for the study of intracellular environments. We report a new class of luminescent rotors, based on the sensitized emission of a terbium(III) complex. A 4-fold increase in both quantum yield and luminescence lifetime was observed in viscous media for the studied complexes, with a lifetime ranging from 0.23 to 0.89 ms over a broad range of viscosities (0.6-1200 cP). The presented approach, relying on the millisecond-scale luminescence lifetime of the lanthanide ions, was applied to fixed T24 cancer cells using temporal sampling lifetime imaging microscopy.