THE molecular ion H3 + occupies a central position in theoretical models of interstellar chemistry1,2. It forms readily in hydrogen-rich interstellar gas clouds when ionized H2 reacts with neutral H2. The H3 + ion can then donate a proton to oxygen, carbon and other heavy atoms. From this beginning nearly 100 different inter-stellar molecules are formed, such as the reactive hydroxyl radical OH, neutral CO, ethanol, the linear polyacetylenes HCxN and cyclic species such as cyclopropenylidene, C3H3. But in spite of a decade of searching3,4, H3 + has eluded detection except in the hydrogen-rich atmosphere of Jupiter5. Here we present evidence for the presence of H3 + in the envelope of supernova 1987A, where it is produced by an unusual chemistry in which excited hydrogen atoms are the main source of molecule formation. Infrared spectra of SN1987A (ref. 6) in the first few hundred days after the explosion contain two previously unidentified peaks which we attribute to H3 +. Chemical modelling produces quantities of H3 + consistent with the observed peak intensities, and also predicts significant amounts of HeH+, which may be responsible for some weaker features in the spectra.