Anorthosite (/əˈnɔːrθəsaɪt/) is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

Anorthosites are of enormous geologic interest, because it is still not fully understood how they form. Most models involve separating plagioclase crystals based on their density. Plagioclase crystals are usually less dense than magma; so, as plagioclase crystallizes in a magma chamber, the plagioclase crystals float to the top, concentrating there.

Anorthosite on Earth can be divided into five types:

Of these, the first two are the most common. These two types have different modes of occurrence, appear to be restricted to different periods in Earth's history, and are thought to have had different origins.

Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the subject of much research.

The presence of Martian anorthosites has also been confirmed and is the subject of on-going research.

Proterozoic anorthosites were emplaced during the Proterozoic Eon (c. 2,500–542 Ma), though most were emplaced between 1,800 and 1,000 Ma.

Proterozoic anorthosites typically occur as extensive stocks or batholiths. The areal extent of anorthosite batholiths ranges from relatively small (dozens or hundreds of square kilometers) to nearly 20,000 km² (7,700 sq mi), in the instance of the Nain Plutonic Suite or Mistastin crater in northern Labrador, Canada.