In this Letter the decay Bs -> J/\psi K+ K- is investigated. Although the J/\psi \phi\ channel is well known, final states at higher K+K- masses have not previously been studied. In the K+K- mass spectrum we observe a significant signal in the f'2(1525) region as well as a non-resonant component. The ratio of the rate in a K+K- mass window of +/- 125 MeV around the f'2 relative to that in the J/\psi \phi\ decay, where \phi -> K+K-, is R=(19.4 +/- 1.8 +/- 1.0)%. Our sample consists of 0.16/fb collected with the LHCb detector using 7 TeV pp collisions.