Aqueous solutions of methyl viologen (MV2+) exhibit anomalous fluorescence behavior. Although it has long fluorescence lifetimes in polar solvents such as acetonitrile, MV2+ has a short fluorescence lifetime in water. Recent experiments by Kohler and co-workers (Henrich et al. J. Phys. Chem. B 2015, 119, 2737-2748) have implicated an excited-state acid/base reaction as the source of the nonradiative decay pathway. While many chemical species exhibit enhanced Bronsted acidity in their excited state, MV2+ is the first example of a species with enhanced Lewis acidity. Using a complete active space configuration interaction (CASCI) approach, excited-state molecular dynamics simulations of aqueous MV2+ are performed in order to test the hypothesis that MV2+ acts as a Lewis photoacid and to elucidate a mechanism for this behavior. These simulations show that the Lewis acidity of MV2+ is indeed enhanced by photoexcitation. On its S-1 excited state, MV2+ reacts with water to generate a hydronium ion approximately 1.5 ps after excitation. After the hydronium ion is produced, the corresponding hydroxide ion adds to MV2+ to form a covalently bound photoproduct and, subsequently, evolves toward a conical intersection.