Combined spectroscopic and ab initio studies of the H2---HN2+ proton-bound complex are presented. Infrared spectra of mass-selected H2---HN2+ complexes in the 2500-4200 cm-1 range display a number of vibrational bands, assigned as due either to the H---H stretch vibration, or tentatively to the N---H stretch in combination with intermolecular stretch and bend motions. Due to subpicosecond energy redistribution, almost all rotational structure is obscured by lifetime broadening. The ab initio calculations conducted at the QCISD(T)/6-311G(2df,2pd) level indicate that the complex is composed of largely undistorted H2 and HN2+ subunits, and has a T-shaped minimum energy geometry with an H2···HN2+ intermolecular bond length of approximately 1.44 Å. Both spectral and theoretical results show that combining the H2 and HN2+ molecules drastically reduces the frequency of the N---H stretch vibration, although the H2 stretch frequency is only modestly affected.