  We report the discovery of a brown dwarf companion to the young M dwarf 1RXS
J235133.3+312720 as part of a high contrast imaging search for planets around
nearby young low-mass stars with Keck-II/NIRC2 and Subaru/HiCIAO. The 2.4"
(~120 AU) pair is confirmed to be comoving from two epochs of high resolution
imaging. Follow-up low- and moderate-resolution near-infrared spectroscopy of
1RXS J2351+3127 B with IRTF/SpeX and Keck-II/OSIRIS reveals a spectral type of
L0$^{+2}_{-1}$. The M2 primary star 1RXS J2351+3127 A exhibits X-ray and UV
activity levels comparable to young moving group members with ages of ~10-100
Myr. UVW kinematics based the measured radial velocity of the primary and the
system's photometric distance (50 +/- 10 pc) indicate it is likely a member of
the ~50-150 Myr AB Dor moving group. The near-infrared spectrum of 1RXS
J2351+3127 B does not exhibit obvious signs of youth, but its H-band morphology
shows subtle hints of intermediate surface gravity. The spectrum is also an
excellent match to the ~200 Myr M9 brown dwarf LP 944-20. Assuming an age of
50-150 Myr, evolutionary models imply a mass of 32 +/- 6 Mjup for the
companion, making 1RXS J2351+3127 B the second lowest-mass member of the AB Dor
moving group after the L4 companion CD-35 2722 B and one of the few benchmark
brown dwarfs known at young ages.
