A search for B+ → Ds +K+K− decays is performed using pp collision data corresponding to an integrated luminosity of 4.8 fb−1, collected at centre-of-mass energies of 7, 8 and 13 TeV with the LHCb experiment. A significant signal is observed for the first time and the branching fraction is determined to be ℬ(B+→Ds+K+K−)=(7.1±0.5±0.6±0.7)×10−6,where the first uncertainty is statistical, the second systematic and the third due to the uncertainty on the branching fraction of the normalisation mode B+→Ds+D¯0. A search is also performed for the pure annihilation decay B+ → Ds +ϕ. No significant signal is observed and a limit of ℬ(B+→Ds+ϕ)<4.9×10−7(4.2×10−7) is set on the branching fraction at 95% (90%) confidence level.[Figure not available: see fulltext.].