  We present the discovery of a 0.2" companion to the young M dwarf GJ 3629 as
part of our high contrast adaptive optics imaging search for giant planets
around low-mass stars with the Keck-II and Subaru telescopes. Two epochs of
imaging confirm the pair is co-moving and reveal signs of orbital motion. The
primary exhibits saturated X-ray emission, which together with its UV
photometry from GALEX point to an age younger than ~300 Myr. At these ages the
companion lies below the hydrogen burning limit with a model-dependent mass of
46 +/- 16 Mjup based on the system's photometric distance of 22 +/- 3 pc.
Resolved YJHK photometry of the pair indicates a spectral type of M7 +/- 2 for
GJ 3629 B. With a projected separation of 4.4 +/- 0.6 AU and an estimated
orbital period of 21 +/- 5 yr, GJ 3629 AB is likely to yield a dynamical mass
in the next several years, making it one of only a handful of brown dwarfs to
have a measured mass and an age constrained from the stellar primary.
