Jarosite is a basic hydrous sulfate of potassium and iron with a chemical formula of KFe₃(OH)₆(SO₄)₂. This mineral is formed in ore deposits by the oxidation of iron sulfides. It was originally found in 1852 by August Breithaupt in the Barranco del Jaroso in the Sierra de la Almagrera (near Los Lobos, Cuevas del Almanzora, Almería, Spain).

Jarosite is hexagonal and brittle, with basal cleavage, a hardness of 2.5-3.5, and a specific gravity of 3.15-3.26. It is translucent to opaque with a vitreous to dull lustre, and is coloured dark yellow to yellowish-brown. It can sometimes be confused with limonite or goethite with which it commonly occurs in the gossan (oxidized cap over an ore body). Jarosite is an iron analogue of the potassium aluminium sulfate, alunite.

In 2004 Jarosite was discovered on Mars by the MER-B rover, which has been interpreted as strong evidence that Mars once possessed large amounts of liquid water.