Titanium dioxide (TiO2) is one of the most common photosensitive materials used in photocatalysis, solar cells, self-cleaning coatings, and sunscreens. Although the crystalline TiO2 phases such as anatase and rutile are well-known to be photoactive, whether amorphous TiO2 is active in photocatalytic reactions is still controversial. Here we show that amorphous TiO2 prepared by the commonly used sol-gel method of tetrabutyl titanate hydrolysis is active in photocatalytic water reduction and methylene blue oxidation under the irradiation of a xenon lamp. The amorphous TiO2 gains photoactivity after an induction period of approximately an hour, suggesting that phase transition is involved. Using an extensive series of microscopic and spectroscopic analyses, we further show that the photoinduced crystallization by amorphous TiO2 forms a nanometer-thin layer of rutile nanocrystallites under the irradiation in the middle ultraviolet range. The resulting core-shell nanoparticles have a bandgap of 3.3 eV and are enriched with surface-active sites including reduced titanium and oxygen vacancies. The revelation of photoinduced crystallization raises the possibility of preparing photosensitive TiO2 using low-temperature radiation techniques that can not only save energy but also incorporate heat-sensitive components into manufacturing. (Graph Presented).