none659siThe decay Bc+ → J/ψ K+ K− π+ is observed for the first time, using proton-proton collisions collected with the LHCb detector corresponding to an integrated luminosity of 3 fb^−1. A signal yield of 78 ± 14 decays is reported with a significance of 6.2 standard deviations. The ratio of the branching fraction of Bc+ → J/ψ K+ K− π+ decays to that of Bc+ → J/ψ π+ decays is measured to be 0.53 ± 0.10 ± 0.05, where the first uncertainty is statistical and the second is systematic.openR. Aaij; B. Adeva; M. Adinolfi; C. Adrover; A. Affolder; Z. Ajaltouni; J. Albrecht; F. Alessio; M. Alexander; S. Ali; G. Alkhazov; P. Alvarez Cartelle; A. A. Alves Jr; S. Amato; S. Amerio; Y. Amhis; L. Anderlini; J. Anderson; R. Andreassen; J. E. Andrews; R. ...