The first observation of the decays Λb0→χc1pK− and Λb0→χc2pK− is reported using a data sample corresponding to an integrated luminosity of 3.0 fb−1, collected by the LHCb experiment in pp collisions at center-of-mass energies of 7 and 8 TeV. The following ratios of branching fractions are measured: B(Λb0→χc1pK−)B(Λb0→J/ψpK−)=0.242±0.014±0.013±0.009,B(Λb0→χc2pK−)B(Λb0→J/ψpK−)=0.248±0.020±0.014±0.009,B(Λb0→χc2pK−)B(Λb0→χc1pK−)=1.02±0.10±0.02±0.05,where the first uncertainty is statistical, the second systematic, and the third due to the uncertainty on the branching fractions of the χc1→J/ψγ and χc2→J/ψγ decays. Using both decay modes, the mass of the Λb0 baryon is also measured to be mΛb0=5619.44±0.28±0.26 MeV/c2, where the first and second uncertainties are statistical and systematic, respectively.Received 27 April 2017DOI:https://doi.org/10.1103/PhysRevLett.119.062001Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2017 CERN, for the LHCb CollaborationPhysics Subject Headings (PhySH)Research AreasBranching fractionParticle decaysQuantum chromodynamicsPhysical SystemsBaryonsBottom quarkTechniquesHadron collidersMachine learningParticles & Fields