The doubly charmed baryon decay Ξ + + c c → Ξ + c π + is observed for the first time, with a statistical significance of 5.9 σ , confirming a recent observation of the baryon in the Λ + c K − π + π + final state. The data sample used corresponds to an integrated luminosity of 1.7     fb − 1 , collected by the LHCb experiment in p p collisions at a center-of-mass energy of 13 TeV. The Ξ + + c c mass is measured to be 3620.6 ± 1.5 ( stat ) ± 0.4 ( syst ) ± 0.3 ( Ξ + c )     MeV / c 2 and is consistent with the previous result. The ratio of branching fractions between the decay modes is measured to be B ( Ξ + + c c → Ξ + c π + ) × B ( Ξ + c → p K − π + ) / B ( Ξ + + c c → Λ + c K − π + π + ) × B ( Λ + c → p K − π + ) = 0.035 ± 0.009 ( stat ) ± 0.003 ( syst )