  We announce the identification of a proper motion companion to the star HII
1348, a K5V member of the Pleiades open cluster. The existence of a faint point
source 1.1arcsec away from HII 1348 was previously known from adaptive optics
imaging by Bouvier et al. However, because of a high likelihood of background
star contamination and in the absence of follow-up astrometry, Bouvier et al.
tentatively concluded that the candidate companion was not physically
associated with HII 1348. We establish the proper motion association of the
pair from adaptive optics imaging with the Palomar 5m telescope. Adaptive
optics spectroscopy with the integral field spectrograph OSIRIS on the Keck 10m
telescope reveals that the companion has a spectral type of M8\pm1. According
to substellar evolution models, the M8 spectral type resides within the
substellar mass regime at the age of the Pleiades. The primary itself is a
known double-lined spectroscopic binary, which makes the resolved companion,
HII 1348B, the least massive and widest component of this hierarchical triple
system and the first substellar companion to a stellar primary in the Pleiades.
