A noninvasive method is introduced for quantification and visualization of fluence rate in light-irradiated biological tissues. The method is based on magnetic resonance thermometry (MRT) measurements of tissue temperature changes resulting from absorption of light. From the spatial–temporal temperature data, the generated heat is calculated. Finally, fluence rate maps are reconstructed by dividing the heat data by the tissue absorption coefficient. Simulations were performed using virtual MRT datasets based on analytically described fluence rate distributions, which could be accurately reconstructed by the method. Next, the approach was tested in gel phantoms. Resulting fluence rate maps matched well with theoretical predictions in a nonscattering phantom (). Experimental validation was further obtained in a scattering phantom, by comparing fluence rates to invasive fluence rate probe measurements along and perpendicular to the optical axis ( for both cases). Finally, our technique was applied in vivo in a mouse tumor model. The resulting fluence rates matched invasive probe measurements (Pearson’s , ). The method may be applied to investigate the relation between light dose and biological response in light-based treatments, such as photodynamic therapy. It may also be useful for experimental validation of light transport models.