We present the discovery of an L0 companion to the nearby M1.5 dwarf G 239-25, at a projected distance of 31 AU. It is the faintest companion discovered so far in our adaptive optics survey of all known M dwarfs within 12 pc, and it lies at the stellar/substellar limit. Given the assumed age of the primary star, the companion is likely an extremely low mass star. The long orbital period of G 239-25 AB (≈100 years) precludes a direct mass determination, but the relatively wide angular separation will allow detailed analyses of its near infrared and visible spectra. Based on observations made at Canada-France-Hawaii Telescope, operated by the National Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii, at the Observatoire de Haute Provence, operated by the Centre National de la Recherche Scientifique de France and at the William Herschel Telescopes operated by the Isaac Newton Group at the Instituto de Astrofísica de Canarias.