We report the observation and measurement of the mass of the bottom, strange baryon Ξ−b through the decay chain Ξ−b→J/ψΞ−, where J/ψ→μ+μ−, Ξ−→Λπ−, and Λ→pπ−. A signal is observed whose probability of arising from a background fluctuation is 6.6×10−15, or 7.7 Gaussian standard deviations. The Ξ−b mass is measured to be 5792.9±2.5(stat) ±1.7(syst)  MeV/c2.