THE H+3 ion is widely believed to play an important role in interstellar chemistry, by initiating the chains of reactions that lead to the production of many of the complex molecular species observed in the interstellar medium1-5. The presence of H+3 in the interstellar medium was first suggested6 in 1961, and its infrared spectrum was measured7 in the laboratory in 1980. But attempts8-11 to detect it in interstellar space have hitherto proved unsuccessful. Here we report the detection of H+3 absorption in the spectra of two molecular clouds. Although the present results do not permit an accurate determination of the H+3 abundances, these ions appear nevertheless to be present in sufficient quantities to drive much of the chemistry in molecular clouds. It should soon be possible to obtain more accurate measurements, and thus better quantify the role of ion-neutral reactions in the chemical evolution of molecular clouds.