The water dimer cation, (H2O)(2)(+), has long served as a prototypical reference system for water oxidation chemistry. In spite of this status, a,definitive explanation for the anomalous-and dominant-features,in the experimental vibrational spectrum [Gardenier, G. H.; Johnson, M. A.; McCoy, A.B. J. Phys. Chem. A, 2009, 113, 4772-4779] has not been determined, and harmonic analyses qualitatively fail to reproduce these features. In this computational study, accurate quantum chemistry methods are combined with a fully coupled, six-dimensional anharmonic model to show that the unassigned-bands are the result of resonant mode interactions and strong anharmonic coupling. Such coupling is fundamentally due to the unique electronic. structure of this open-shell ion and the manner in which auxiliary modes affect the natural charge-transfer properties of the shared-proton stretch. These unique vibrational signatures provide a key reference point for modern spectroscopic and mechanistic analyses of water-oxidation catalysts.