Single particle spectroscopy in the form of three-dimensional optical manipulation of an upconverting nanoparticle is here used for non-invasive thermal sensing at the cellular level. In particular, a single infrared 980 nm laser beam is used as a three-dimensional optical tweezer and, simultaneously, as an optical excitation source for a single NaYF4:Er3+,Yb3+ upconverting particle. Real time analysis of the thermosensitive green emission of Er3+ ions obtained after Yb3+ excitation provides thermal sensing during optical manipulation. Thus, three-dimensional particle scanning allows for the measurement of thermal gradients in the surroundings of individual cancer cells subjected to a plasmonic-mediated photothermal therapy. It is found that such thermal gradients extends for distances larger than 10 microns, avoiding real single cell photothermal treatments under in vitro conditions. This work introduces to the scientific community a novel and simple approach for high resolution thermal sensing at the cellular level that could constitute a powerful tool for a better understanding of cell dynamics during thermal treatments