The non-equilibrium beta-Bi2O3 polymorph is stabilized in thin films by a photochemical synthesis method. A strong ultraviolet-absorbing bismuth(III)-N-methyldiethanolamine complex is synthesized in solution as an ideal precursor for the beta-Bi2O3 phase. Ultraviolet-light induces the formation of an amorphous -Bi-O-Bi- continuous network in the films deposited from the former solution that easily converts into the beta-Bi2O3 polymorph at a temperature as low as 250 A degrees C. The room temperature stabilization of the beta-Bi2O3 phase is confirmed by their structural characterization using four-circle X-ray diffractometry. This study unequivocally identified the tetragonal crystal structure of the beta-Bi2O3 polymorph in the films. The high phase purity of these beta-Bi2O3 films is responsible for their exceptional visible-light photocatalytic activity, thus enabling the applications of the films of this metastable phase at room-temperature conditions.