Structural properties of the B+−H2 electrostatic complex are investigated through its rotationally resolved infrared spectrum in the H−H stretch region (3905−3975 cm−1). The spectrum, which was obtained by monitoring B+ photofragments while the IR wavelength was scanned, is consistent with the complex having a T-shaped structure and a vibrationally averaged intermolecular separation of 2.26 Å, which decreases by 0.04 Å when the H2 subunit is vibrationally excited. The H−H stretch transition of B+−H2 is red-shifted by 220.6 ± 1.5 cm−1 from that of the free H2 molecule, much more than for other dihydrogen complexes with comparable binding energies. Properties of B+−H2 and the related Li+−H2, Na+−H2, and Al+−H2 complexes are explored through ab initio calculations at the MP2/aug-cc-pVTZ level. The unusually large red-shift for B+−H2 is explained as due to electron donation from the H2 σg bonding orbital to the unoccupied 2pz orbital on the B+ ion.