BROWN dwarfs are star-like objects with masses less than 0.08 times that of the Sun, which are unable to sustain hydrogen fusion in their interiors(1-4). They are very hard to detect, as most of the energy of gravitational contraction is radiated away within similar to 10(8) yr, leaving only a very low residual luminosity. Accordingly, almost all searches for brown dwarfs have been directed towards clusters of young stars-a strategy that has recently proved successful(5,6). But there are only modest observable differences between young brown dwarfs and very low-mass stars, making it difficult to identify the former without appealing to sophisticated models(7). Older brown dwarfs should have a more distinctive appearance, and if they are companions to nearby stars, their luminosity can be determined unambiguously. Here we report the discovery of a probable companion to the nearby star GI229, with no more than one-tenth the luminosity of the least luminous hydrogen-burning star. We conclude that the companion, GI229B, is a brown dwarf with a temperature of less than 1,200 K, acid a mass similar to 20-50 times that of Jupiter.