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Goethite is an iron oxyhydroxide mineral with the chemical formula FeO(OH). It is a common constituent of rocks and soils, and it is also found in meteorites. Goethite is named after Johann Wolfgang von Goethe, a German poet and scientist.

Goethite is a member of the oxohydroxide group, which also includes lepidocrocite and akaganeite. It crystallizes in the orthorhombic system and typically forms as a pseudomorph after other iron minerals, such as hematite and magnetite. It can also form directly through the oxidation of ferrous iron minerals in the presence of water.

The mineral is typically brown to black in color and has a fibrous or acicular habit. It is relatively soft, with a Mohs hardness of 5 to 5.5. Goethite is insoluble in water but dissolves in strong acids.

Goethite is an important ore of iron and is also used in pigments. It is found in many locations around the world, including Brazil, Australia, and the United States. It is also a common weathering product of iron-bearing rocks and minerals. In some environments, goethite can form as concretions or nodules, such as in bog iron deposits. Its presence in soils can contribute to their reddish-brown coloration. Goethite is also known to form under various conditions, including hydrothermal and low-temperature aqueous environments. Its stability range makes it a widespread mineral in Earth's crust.