Magnetically induced acceleration of a photocatalytic reaction was observed for the first time. Upon irradiation of ultrafine TiO2 particles in tert-butyl alcohol at room temperature, the decomposition reaction of tert-butyl alcohol generated acetone and methane as the main products. The yield of acetone was found to increase with increasing magnetic field from 0 to 1.5 T. An increase of about 10% in the yield was observed at 1.5 T. The observed magnetic field effects can be explained by the magnetically induced blocking of the recombination of electrons and holes in the semiconductor.