An observation is reported of the electroweak production of a W+W- pair in association with two jets, with both W bosons decaying leptonically. The data sample corresponds to an integrated luminosity of 138 fb-1 of proton-proton collisions at √s= 13 TeV, collected by the CMS detector at the CERN LHC. Events are selected by requiring exactly two opposite-sign leptons (electrons or muons) and two jets with large pseudorapidity separation and high dijet invariant mass. Events are categorized based on the flavor of the final-state leptons. A signal is observed with a significance of 5.6 standard deviations (5.2 expected) with respect to the background-only hypothesis. The measured fiducial cross section is 10.2 +/- 2.0 fb and this value is consistent with the standard model prediction of 9.1 +/- 0.6 fb. The inclusive fiducial cross section is defined with LHE level requirements: pT(q) > 10 GeV m(qq) > 100 The exclusive fiducial cross section is defined with GEN level requirements: emu, ee or mumu opposite-charge final state Dressed leptons with pT(l) > 25,13 GeV, |eta(l)| < 2.5 Third lepton veto if pT(l) > 10 GeV m(ll) > 120 GeV, pT(l) > 30 GeV At least 2 jets, no b-jets pT(j) > 30 GeV, |eta(j)| < 4.7 DR(l, j) > 0.4 m(jj) > 300 GeV, Deta(jj) > 2.5 pTmiss > 30 GeV