We measure the decay Bs0→K0K0 using data collected at the ?(5S) resonance with the Belle detector at the KEKB e+e- collider. The data sample used corresponds to an integrated luminosity of 121.4 fb-1. We measure a branching fraction B(Bs0→K0K0)=[19.6-5.1+5.8(stat)±1.0(syst)±2.0(NBs0Bs0)]×10-6 with a significance of 5.1 standard deviations. This measurement constitutes the first observation of this decay.