We present an observation for ZZ→ℓ+ℓ−ℓ'+ℓ'− (ℓ, ℓ′=e or μ) production in p¯p collisions at a center-of-mass energy of √s=1.96 TeV. Using 1.7 fb−1 of data collected by the D0 experiment at the Fermilab Tevatron Collider, we observe three candidate events with an expected background of 0.14+0.03−0.02 events. The significance of this observation is 5.3 standard deviations. The combination of D0 results in this channel, as well as in ZZ→ℓ+ℓ−ν¯ν, yields a significance of 5.7 standard deviations and a combined cross section of σ(ZZ)=1.60±0.63(stat)+0.16−0.17(syst) pb.Received 6 August 2008DOI:https://doi.org/10.1103/PhysRevLett.101.171803©2008 American Physical Society