Acid-sulfate minerals such as jarosite are often used as indicators of environmental conditions on Mars. These minerals form in a diverse set of environments on Earth; however, all known Martian jarosite deposits are most likely evaporative, from evaporation of either groundwater or ponded water. Here, we use the Compact Reconnaissance Imaging Spectrometer for Mars and High-Resolution Imaging Science Experiment to report a new jarosite deposit along the southern wall of Ius Chasma. The morphology and geologic context of the Ius deposit are unique on Mars, and difficult to explain with an evaporative or groundwater mechanism. We propose instead that it was deposited along the margins of a past glacier. Such acid-ice interactions would be similar to those reported along the margins of Svalbard glaciers (arctic Norway), and would represent a new style of acid-sulfate formation on Mars.