Context. The large and vertically non-uniform abundance of CO in Neptune’s atmosphere has been interpreted as the result of past cometary impact(s), either single or distributed in size and time, which could also be at the origin of Neptune’s HCN. Aims. We aim to provide observational support for this scenario by searching for other comet-induced species, in particular carbon sulfide (CS) which has been observed continuously in Jupiter since the 1994 Shoemaker-Levy 9 impacts. Methods. In April 2016 we used the ALMA interferometer to search for CS(7-6) at 342.883 GHz in Neptune. Results. We report on the detection of CS in Neptune’s atmosphere, the first unambiguous observation of a sulfur-bearing species in a giant planet beyond Jupiter. Carbon sulfide appears to be present only at submillibar levels, with a column density of (2.0–3.1) × 1012 cm-2, and a typical mixing ratio of (2−20) × 10-11 that depends on its precise vertical location. The favoured origin of CS is deposition by a putative large comet impact several centuries ago, and the strong depletion of CS with respect to CO – compared to the Jupiter case – is likely due to the CS sticking to aerosols or clustering to form polymers in Neptune’s lower stratosphere. Conclusions. The CS detection, along with recent analyses of the CO profile, reinforces the presumption of a large comet impact into Neptune ~1000 yr ago, that delivered CO, CS, and HCN at the same time.