A model for the realization of optical bistability in thin polymeric photochromic film is proposed. According to the model, an illuminated photochromic polymeric system can demonstrate optical bistability under nonisothermal conditions. This bistability arises due to photothermal reactions of the photochromic compound, resulting in an increase of the absorption of radiation and establishing positive feedback between the absorption and temperature. It is shown that bistability can be observed upon changing either the incident laser intensity or the frequency of the radiation. The influence of the properties of the system on the bistability is studied, and critical parameters for its realization are determined.