The first observation of the decay B0→D0K+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 B0→D0π+π- to be B(B0→D0K+K-)/B(B0→D0π+π-)=0.056±0.011±0.007, where the first uncertainty is statistical and the second is systematic. The significance of the signal is 5.8σ. Evidence, with 3.8σ significance, for Bs0→D0K+K- decays is also presented. The relative branching fraction is measured to be B(Bs0→D0K+K-)/B(B0→D0K+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
