The equilibrium structure and vibrational frequencies of the nitrate cation, NO3+, have been investigated with an extensive set of ab initio calculations. Two stationary points were identified on the NO3+ potential energy surface: a symmetric D-3h structure and a C-2 upsilon ring structure similar to that found for the isoelectronic CO3 molecule. Geometry optimizations executed at the CCSD(T)/aug-cc-pVTZ level of theory yielded the following data. NO3+(D-3h): E-rel = 2130 cm(-1), r(e)= 1.238 Angstrom. NO3+(C-2 upsilon): E-rel = 0 cm(-1), r(1) = 1.131 Angstrom, r(2) = r(3) = 1.309 Angstrom, theta = 142.3 degrees. Calculations performed at the B3LYP, QCISD, CCSD, and CCSD(T) levels of theory all predict the C-2 upsilon, structure to be lower in energy than the D3h structure, Relative energy calculations performed with the Gaussian and complete basis set model chemistry algorithms also predict the C-2 upsilon structure to be the most stable NO3+ conformation. These results are supported by vibrational frequency calculations which suggest that the D-3h structure may correspond to a second-order saddle point rather than a true minimum on the NO3+ potential energy hypersurface. The symmetry breaking observed in the present NO3+ calculations is similar to that observed in ab initio studies of the NO3 equilibrium structure and is used to examine symmetry breaking across the nitrate series NO3-, NO3, NO3+.