The top quark pair production cross section is measured for the first time in proton-proton collisions at $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$ by the CMS experiment at the CERN LHC, using data corresponding to an integrated luminosity of $43\text{ }\text{ }{\mathrm{pb}}^{\ensuremath{-}1}$. The measurement is performed by analyzing events with at least one electron and one muon of opposite charge, and at least two jets. The measured cross section is $746\ifmmode\pm\else\textpm\fi{}58(\text{stat})\ifmmode\pm\else\textpm\fi{}53(\text{syst})\ifmmode\pm\else\textpm\fi{}36(\text{lumi})\text{ }\text{ }\mathrm{pb}$, in agreement with the expectation from the standard model.