Infrared photodissociation spectra of buffer-gas-cooled [MgNO3(H2O)N-n](+) complexes with n = 1-4 are measured in the O-H stretching region. The observed bands are assigned to the excitation of the symmetric and antisymmetric stretching modes of the water molecules. The structural assignment of the spectra is aided by density functional theory calculations (B3LYP/6-311+G(d,p)) on energetically low-lying isomers, including the calculation of harmonic and anharmonic vibrational frequencies, as well as dissociation energies. The nitrate anion binds to the Mg dication in a bidentate fashion, occupying two coordination sites. The water molecules fill the remaining binding sites of the Mg cation, completing the first coordination shell at n = 4 and forming a stable six-fold-coordinated complex, the structure of which persists up to room temperature.