We repport 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 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 micron respectively. For the physical conditions of the different absorbing clouds the H2D^+ column density ranges from 2 to 5*1013 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 20 K.