Accepted in Astrophysics Journal Letters. Non edited.We report on the first far-IR detection of H2D+, using the Infrared Space Observatory, in the line of sight toward Sgr B2 in the galactic center. The transition at λ=126.853 μm connecting the ground level of o-H2D+, 1_1,1 with the the 2_1,2 level at 113 K, is observed in absorption against the continuum emission of the cold dust of the source. The line is broad, with a total absorption covering 350 km s^-1, i.e., similar to that observed in the fundamental transitions of H2O, OH and CH at ~179, 119 and 149 μm respectively. For the physical conditions of the different absorbing clouds the H2D+ column density ranges from 2 to 5x10^13 cm^-2, i.e., near an order of magnitude below the upper limits obtained from ground based submillimeter telescopes. The derived H2D+ abundance is of a few 10^-10, which agrees with chemical models predictions for a gas at a kinetic temperature of ~20K.We thank the Spanish MEC for funding support through grant ESP2004-665, AYA2003-2785, and "Comunidad de Madrid" Government under PRICIT project S-0505/ESP-0237 (ASTROCAM). This study is supported in part by the European Community’s human potential Programme under contract MCRTN-CT-2004-51230, Molecular Universe. JRG was supported by an individual Marie Curie fellowship under contract MEIFCT-2005-5153340.Peer reviewed