We report the discovery of singly ionized chlorine in the Io plasma torus through the detection of emission at 857.9 nm. The 40 Rayleigh peak intensity at 857.9 nm corresponds to a Cl+ abundance of 2¿0.5% relative to all ions. Since Cl+ probably originates at Io, Io's atmosphere must have the highest proportion of chlorine of any solar system body. While Cl+ plays only a minor role in torus physics, the presence of chlorine in Io's atmosphere, surface, and interior has more profound implications. Salt as a solid or gas is probably an important source of both Na and Cl in Io's vicinity. ¿ 2000 American Geophysical Union