Alpha Centauri (α Centauri, α Cen, or Alpha Cen) is a star system in the southern constellation of Centaurus. It consists of three stars: Rigil Kentaurus (α Centauri A), Toliman (α Centauri B), and Proxima Centauri (α Centauri C). Proxima Centauri is the closest star to the Sun at 4.2465 light-years (ly), which is 1.3020 parsecs (pc), while Alpha Centauri A and B are the nearest stars visible to the naked eye.

Rigil Kentaurus and Toliman are Sun-like stars (class G and K, respectively) that together form the binary star system α Centauri AB. To the naked eye, these two main components appear to be a single star with an apparent magnitude of −0.27. It is the brightest star in the constellation and the third-brightest in the night sky, outshone by only Sirius and Canopus. α Centauri AB are the nearest binary stars to the Sun at a distance of 4.344 ly (1.33 pc).

Rigil Kentaurus has 1.1 times the mass (M_☉) and 1.5 times the luminosity of the Sun (L_☉), while Toliman is smaller and cooler, at 0.9 M_☉ and less than 0.5 L_☉. The pair orbit around a common centre with an orbital period of 79 years. Their elliptical orbit is eccentric, so that the distance between A and B varies from 35.6 astronomical units (AU), or about the distance between Pluto and the Sun, to 11.2 AU, or about the distance between Saturn and the Sun.

Proxima Centauri is a small faint red dwarf (class M). Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun at a distance of 4.24 ly (1.30 pc), slightly closer than α Centauri AB. The distance between Proxima Centauri and α Centauri AB is about 13,000 AU (0.21 ly), equivalent to about 430 times the radius of Neptune's orbit.

Proxima Centauri has two confirmed planets — Proxima b and Proxima d. The former is an Earth-sized planet in the habitable zone (though it is unlikely to be habitable) while the latter is a sub-Earth which orbits very closely to the star. A possible but disputed third planet, Proxima c, is a mini-Neptune 1.5 astronomical units away. Rigil Kentaurus may have a Saturn-mass planet in the habitable zone, though it is not yet known with certainty to be planetary in nature. Toliman has no known planets.

α Centauri (Latinised to Alpha Centauri) is the system's designation given by J. Bayer in 1603. It belongs to the constellation Centaurus, named after the part human, part horse creature in Greek mythology; Heracles accidentally wounded the centaur and placed him in the sky after his death. Alpha Centauri marks the right front hoof of the Centaur. The common name Rigil Kentaurus is a Latinisation of the Arabic translation رجل القنطورس Rijl al-Qinṭūrus, meaning "the Foot of the Centaur". Qinṭūrus is the Arabic transliteration of the Greek Κένταυρος (Kentaurus). The name is frequently abbreviated to Rigil Kent (/ˈraɪdʒəl ˈkɛnt/) or even Rigil, though the latter name is better known for Rigel (β Orionis).

An alternative name found in European sources, Toliman, is an approximation of the Arabic الظليمان aẓ-Ẓalīmān (in older transcription, aṭ-Ṭhalīmān), meaning 'the (two male) Ostriches', an appellation Zakariya al-Qazwini had applied to the pair of stars Lambda and Mu Sagittarii; it was often unclear on old star maps which name was intended to go with which star (or stars), and the referents changed over time. The name Toliman originates with Jacob Golius' 1669 edition of Al-Farghani's Compendium. Tolimân is Golius' Latinisation of the Arabic name الظلمان al-Ẓulmān "the ostriches", the name of an asterism of which Alpha Centauri formed the main star.

α Centauri C was discovered in 1915 by Robert T. A. Innes, who suggested that it be named Proxima Centaurus, from Latin 'the nearest [star] of Centaurus'. The name Proxima Centauri later became more widely used and is now listed by the International Astronomical Union (IAU) as the approved proper name; it is frequently abbreviated to Proxima.