A search for B+ → D + 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 $$ \mathrm{\mathcal{B}}\left({B}^{+}\to {D}_s^{+}{K}^{+}{K}^{-}\right)=\left(7.1 \pm 0.5 \pm 0.6 \pm 0.7\right)\times 1{0}^{-6}, $$ where the first uncertainty is statistical, the second systematic and the third due to the uncertainty on the branching fraction of the normalisation mode $$ {B}^{+}\to {D}_s^{+}{\overline{D}}^0 $$ . A search is also performed for the pure annihilation decay B+ → D + ϕ. No significant signal is observed and a limit of $$ \mathrm{\mathcal{B}}\left({B}^{+}\to {D}_s^{+}\phi \right)<4.9 \times 1{0}^{-7}\left(4.2 \times 1{0}^{-7}\right) $$ is set on the branching fraction at 95% (90%) confidence level.