The top quark pair production cross section is measured for the first time in proton-proton collisions at s=13  TeV by the CMS experiment at the CERN LHC, using data corresponding to an integrated luminosity of 43  pb-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±58(stat)±53(syst)±36(lumi)  pb, in agreement with the expectation from the standard model.