Proxima Centauri (also known as Alpha Centauri C or Alpha Proxima) is the nearest known star to the sun, at a distance of about 4.2 light years (or 40 trillion km). It is a spectral type M intrinsically faint red star, more than ten magnitudes (ten thousand times) fainter than the Sun. It is also much cooler, with a surface temperature of about 3100 C. Its visual (apparent) magnitude is eleven, so it is only visible with a good telescope, and only then from southern latitudes. Proxima is about one-tenth the mass of the sun, which accounts for its low surface temperature. It is possibly an outlying member of the triple alpha Centauri system just a few light days closer to us than the other, much brighter stars in the group.

In Star Trek, Proxima Centauri has a planetary system of its own with a settlement on the Proxima colony. (ENT: "Borderland")
In the 24th century, Proxima Centauri is home to Starfleet's Proxima Maintenance Yards. Under the command of Admiral Drazman in 2371, Commander Benjamin Sisko, Doctor Julian Bashir and Lt. Jadzia Dax planned to visit the Yards in that year. (DS9: "Past Tense, Part I") The Federation starship USS Proxima was named for this system.

In reality, Proxima Centauri may be gravitationally bound to Alpha Centauri A and B, 0.29 light-years (13,000 AU) away, with an orbital period of 0.5 to 2 million years. Alternatively, it may be on an open (hyperbolic) trajectory past Alpha Centauri that will eventually take it away from the system. According to this idea, Proxima is an independent member of a moving group that includes Alpha Cen A and B and a number of other nearby stars.

Because of its nearness, both to the Sun and its Alpha Cen neighbors, Proxima has well-determined physical properties, including a mass of 0.11 solar masses, a radius of about 15 percent that of the Sun, and an age of 5 to 6 billion years. Despite its considerable age, Proxima has an active chromosphere and is also a flare star (variable-star designation V645 Centauri), capable of brightening a magnitude or more in minutes. Observations of its chromosphere at ultraviolet wavelengths suggest a rotation period of about 31 days. Claims made in the mid-1990s, based on data from the Hubble Space Telescope, that Proxima may be orbited by a large planet or a brown dwarf, have not been substantiated.