We report a solid-state 17O NMR study of the 17O electric field gradient (EFG) and chemical shielding (CS) tensors for the oxonium ion, H3O+, in p-toluenesulfonic acid monohydrate (TAM). Both the 17O EFG and CS tensors of the H3O+ ion are axially symmetric within the experimental errors. The 17O quadrupole coupling constant (QCC) is found to be 7.05 ± 0.02 MHz, and the 17O chemical shift anisotropy (CSA) is 87 ± 5 ppm. Experimental results are compared with extensive quantum chemical calculations using restricted Hartree−Fock approach (RHF), second-order Moller−Plesset perturbation theory (MP2), and density functional theory (DFT). The calculations showed that the strong hydrogen-bonding environment around the H3O+ ion in TAM is responsible for a reduction of approximately 3 MHz in the 17O QCC compared to that of an isolated H3O+ ion. The effective 17O quadrupole moment is calibrated at the B3LYP/cc-pVTZ level, Q = −2.400 fm2. Using this value, we obtained the best calculated 17O QCC for the “bound” H3O+ ...