We report the observation of the doubly strange b baryon Ω−b in the decay channel Ω−b→J/ψΩ−, with J/ψ→μ+μ− and Ω−→ΛK−→(pπ−)K−, in p¯p collisions at √s=1.96 TeV. Using approximately 1.3 fb−1 of data collected with the D0 detector at the Fermilab Tevatron Collider, we observe 17.8±4.9(stat)±0.8(syst) Ω−b signal events at a mass of 6.165±0.010(stat)±0.013(syst) GeV. The significance of the observed signal is 5.4σ, corresponding to a probability of 6.7×10−8 of it arising from a background fluctuation.Received 29 August 2008DOI:https://doi.org/10.1103/PhysRevLett.101.232002©2008 American Physical Society