An accurate near-equilibrium potential energy surface (PES) has been constructed for the azide ion (N(3)(-)) on the basis of coupled cluster calculations up to CCSDTQ (Kállay, M.; Surján, P. R. J. Chem. Phys. 2001, 115, 2945.), with contributions from inner-shell correlation and special relativity being taken into account as well. A larger number of rovibrational states has been investigated by variational calculations with Watson's isomorphic Hamiltonian for linear molecules. Analogous calculations for CO2 demonstrate the high quality of this type of calculations. The G(v) values of the symmetric stretching and bending vibration of 14N(3)(-) are predicted to be ν1 = 1307.9 cm(-1) and ν2 = 629.3 cm(-1), with an uncertainty of ca. 1 cm(-1). Fermi resonance is less pronounced for the lower polyads of 14N(3)(-) compared with 12C16O2 but is as strong as in CO2 for the lowest diad of isotopologue 15-14-15. The band origin of the antisymmetric stretching vibration of 14N(3)(-) is calculated to be ν3 = 1986.4 cm(-1), only 0.1 cm(-1) lower than the experimental value. The corresponding vibrational transition dipole moment is predicted to be as large as μ = 0.476 D, 46% higher than calculated for CO2. The perturbed combination tone (01(1)1), which was accessible through diode laser IR spectroscopy, undergoes anharmonic interaction with at least two other vibrational states.