We report the first experimental observation of a magnetic-field induced transition from an Anderson insulator to a quantum Hall conductor in a two-dimensional electron gas (2DEG) system. With sufficiently strong disorder, the 2DEG is an Anderson insulator with a divergent resistance at B = 0. The 2DEG is transformed into a quantum Hall conductor when the lowest Landau level is filled. This magnetic-field-induced delocalization is consistent with a recently proposed global phase diagram for the quantum Hall effect.