  We construct an analytic model for the breakout of a relativistic radiation
mediated shock from a stellar wind, and exploit it to calculate the
observational diagnostics of the breakout signal. The model accounts for photon
escape through the finite optical depth wind, and treats the fraction of
downstream photons escaping to infinity as an adiabatic parameter that evolves
in a quasi-steady manner. It is shown that the shock is mediated by radiation
even when a large fraction of the downstream photons escape, owing to
self-generation and adjustment of opacity through accelerated pair creation.
Relativistic breakout occurs at radii at which the total optical depth of the
wind ahead of the shock is $\sim (m_e/m_p)\Gamma_{sh}$, provided that the local
shock Lorentz factor $\Gamma_{sh}$ exceeds unity at this location. Otherwise
the breakout occurs in the Newtonian regime. A relativistic breakout is
expected in a highly energetic spherical explosion ($10^{52}-10^{53}$ erg) of a
Wolf-Rayet star, or in cases where a smaller amount of energy ($\sim 10^{51}$
erg) is deposited by a jet in the outer layers of the star. The properties of
the emission observed in such explosions during the relativistic breakout are
derived. We find that for typical parameters about $10^{48}$ ergs are radiated
in the form of MeV gamma-rays over a duration that can range from a fraction of
a second to an hour. Such a signal may be detectable out to 10-100 Mpc by
current gamma-ray satellites.
