  We have identified an X-ray transient (hereafter M15 X-3) in the globular
cluster M15 from an archival Chandra grating observation. M15 X-3 appears at an
X-ray luminosity of 6*10^{33} ergs/s with a spectrum consistent with an
absorbed power law of photon index 1.51+-0.14. The object is identifiable in
archival Chandra HRC-I observations with an X-ray luminosity of 2-6*10^{31}
ergs/s and apparently soft colors, suggesting a neutron star low-mass X-ray
binary in quiescence. We also observe it in outburst in a 2007 Chandra HRC-I
observation, and in archival 1994-1995 ROSAT HRI observations.
  We identify a likely optical/UV counterpart with a (possibly transient) UV
excess from archival HST data, which suggests a main sequence companion. We
argue that M15 X-3's behavior is similar to that of the very faint X-ray
transients which have been observed in the Galactic Center. We discuss several
explanations for its very low X-ray luminosity, with the assumption that we
have detected its companion. M15 X-3's uniquely low extinction and
well-determined distance make it an excellent target for future studies.
