The doubly charmed baryon decay ${\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{+}{\ensuremath{\pi}}^{+}$ is observed for the first time, with a statistical significance of $5.9\ensuremath{\sigma}$, confirming a recent observation of the baryon in the ${\mathrm{\ensuremath{\Lambda}}}_{c}^{+}{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{+}$ final state. The data sample used corresponds to an integrated luminosity of $1.7\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of 13 TeV. The ${\mathrm{\ensuremath{\Xi}}}_{cc}^{++}$ mass is measured to be $3620.6\ifmmode\pm\else\textpm\fi{}1.5(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.4(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}0.3({\mathrm{\ensuremath{\Xi}}}_{c}^{+})\text{ }\text{ }\mathrm{MeV}/{c}^{2}$ and is consistent with the previous result. The ratio of branching fractions between the decay modes is measured to be $[\mathcal{B}({\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Xi}}}_{c}^{+}{\ensuremath{\pi}}^{+})\ifmmode\times\else\texttimes\fi{}\mathcal{B}({\mathrm{\ensuremath{\Xi}}}_{c}^{+}\ensuremath{\rightarrow}p{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+})]/[\mathcal{B}({\mathrm{\ensuremath{\Xi}}}_{cc}^{++}\ensuremath{\rightarrow}\phantom{\rule{0ex}{0ex}}{\mathrm{\ensuremath{\Lambda}}}_{c}^{+}{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{+})\ifmmode\times\else\texttimes\fi{}\mathcal{B}({\mathrm{\ensuremath{\Lambda}}}_{c}^{+}\ensuremath{\rightarrow}p{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+})]=0.035\ifmmode\pm\else\textpm\fi{}0.009(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.003(\mathrm{syst})$.