The 692 GHz para ground-state line of D2H+ has been detected at the Caltech Submillimeter Observatory toward the prestellar core 16293E. The derived D2H+ abundance is comparable to that of H2D+, as determined by observations of the 372 GHz line of ortho-H2D+. This is an observational verification of recent theoretical predictions, developed to explain the large deuteration ratios observed in cold, high-density regions of the interstellar medium associated with low-mass prestellar cores and protostars. This detection confirms expectations that the multiply deuterated forms of H+3 were missing factors of earlier models. The inclusion of D2H+ and D+3 in the models leads to predictions of higher values of the D/H ratio in the gas phase.