Hydrogen sulphide (H₂S) is one of the main trace gases released from volcanoes with yearly global emissions estimated between 1 and 37 Tg. With sulfur dioxide (SO₂, 15–21 Tg/year), it dominates the volcanic sulfur budget, and the emission ratio H₂S:SO₂ is an important geochemical probe for studying source conditions, sulfur chemistry and magma-water interactions. Contrary to SO₂, measurements of H₂S are sparse and difficult. Here we report the first measurements of a large H₂S plume from space. Observations were made with the infrared sounder IASI of the volcanic plume released after the 7–8 August 2008 eruption of Kasatochi volcano. The eruption was characterized by 5 consecutive explosive events. The first events were phreatomagmatic producing a plume rich in water vapor and poor in ash and SO₂. We show that the observed H₂S plume, calculated at 29 ± 10 kT with integrated columns exceeding 140 ± 25 Dobson Units (DU), is likely associated with these first explosions. H₂S:SO₂ ratios with maximum values of 12 ± 2 are found, representative of redox conditions in the hydrothermal envelop. With a detection threshold of 25 DU, future space observations of H₂S plumes are certain. These will be important for improving the atmospheric sulfur budget and characterizing the H₂S:SO₂ fingerprint of different eruptions.