We report the observation and measurement of the mass of the
bottom, strange baryon Xi-_b through the decay chain Xi-_b --> J/psi Xi-,
where J/psi --> mu+ mu-, Xi- --> Lambda pi+-, and Lambda --> p pi+-.
Evidence for observation is based on a signal whose probability of arising
from the estimated background is 6.6 x 10-15, or 7.7 Gaussian standard
deviations. The Xi-_b mass is measured to be 5792.9 +/- 2.5 (stat.) +/-
1.7 (syst.) MeV/c2.