We present the discovery of a 0 \farcs 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 \sim 300 Myr .
At these ages the companion lies below the hydrogen burning limit with a model-dependent mass of 46 \pm 16 M _ { \mathrm { Jup } } based on the system ’ s photometric distance of 22 \pm 3 pc .
Resolved YJHK photometry of the pair indicates a spectral type of M7 \pm 2 for GJ 3629 B .
With a projected separation of 4.4 \pm 0.6 AU and an estimated orbital period of 21 \pm 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 .