The nature of interstitial hydrogen in SnO2 has been inferred by a combined study of first-principles theory and infrared absorption, near band edge absorption, and photoluminescence spectroscopy. The earlier interpretation of a center with an O-H stretch mode at 3156 cm-1 as interstitial hydrogen is confirmed. Uniaxial stress experiments on the 3156 cm-1 mode reveal that the interstitial hydrogen atom is located in the open c channel of the rutile SnO2 structure. Migration along the c axis of the crystals occurs via low barrier hydrogen jumps of around 0.57 eV to symmetrically equivalent nearest-neighbor positions. Combinational vibrations of the O-H stretch mode with the out-of-ab-plane and in-ab-plane wag modes at about 4014 and 4332 cm-1 have been identified. Free carrier absorption and excitonic properties of the defect demonstrate that Hi forms an effective shallow-donor-like state similar to the well-characterized F, Cl, and Sb n-type dopants in this material.