Аннотация:The B+→D+sD−sK+ decay is observed for the first time using proton-proton collision data collected by the LHCb detector at centre-of-mass energies of 7, 8 and 13TeV, corresponding to an integrated luminosity of 9fb−1. Its branching fraction relative to that of the B+→D+D−K+ decay is measured to beB(B+→D+sD−sK+)B(B+→D+D−K+)=0.525±0.033±0.027±0.034,where the first uncertainty is statistical, the second systematic, and the third is due to the uncertainties on the branching fractions of the D±s→K∓K±π± and D±→K∓π±π± decays. This measurement fills an experimental gap in the knowledge of the family of Cabibbo−favoured b¯→c¯cs¯ transitions and opens the path for unique studies of spectroscopy in future.