The first observation of the decay B-0 -> (D) over bar (0) K+ K- is reported from an analysis of 0.62 fb(-1) of pp collision data collected with the LHCb detector. Its branching fraction is measured relative to that of the topologically similar decay B-0 -> (D) over bar (0) pi(+) pi(-) to be B(B-0 -> (D) over bar (0) K+ K-)/B(B-0 -> (D) over bar (0) pi(+) pi(-)) = 0.056 +/- 0.011 +/- 0.007; where the first uncertainty is statistical and the second is systematic. The significance of the signal is 5.8 sigma. Evidence, with 3.8 sigma significance, for B-s(0) -> (D) over bar (0) K+ K- decays is also presented. The relative branching fraction is measured to be B(B-s(0) -> (D) over bar (0) K+ K-)/B(B-0 -> (D) over bar (0) K+ K-) = 0.90 +/- 0.27 +/- 0.20. These channels are of interest to study the mechanisms behind hadronic B decays, and open new possibilities for CP violation analyses with larger data sets.