The doubly charmed baryon decay Ξ++cc→Ξ+cπ+ is observed for the first time, with a statistical significance of 5.9σ, confirming a recent observation of the baryon in the Λ+cK−π+π+ final state. The data sample used corresponds to an integrated luminosity of 1.7  fb−1, collected by the LHCb experiment in pp collisions at a center-of-mass energy of 13 TeV. The Ξ++cc mass is measured to be 3620.6±1.5(stat)±0.4(syst)±0.3(Ξ+c)  MeV/c2 and is consistent with the previous result. The ratio of branching fractions between the decay modes is measured to be [B(Ξ++cc→Ξ+cπ+)×B(Ξ+c→pK−π+)]/[B(Ξ++cc→Λ+cK−π+π+)×B(Λ+c→pK−π+)]=0.035±0.009(stat)±0.003(syst)