  A measurement of the cross section of the associated production of a single
top quark and a W boson in final states with a muon or electron and jets in
proton-proton collisions at $\sqrt{s}$ = 13 TeV is presented. The data
correspond to an integrated luminosity of 36 fb$^{-1}$ collected with the CMS
detector at the CERN LHC in 2016. A boosted decision tree is used to separate
the tW signal from the dominant $\mathrm{t\bar{t}}$ background, whilst the
subleading W+jets and multijet backgrounds are constrained using data-based
estimates. This result is the first observation of the tW process in final
states containing a muon or electron and jets, with a significance exceeding 5
standard deviations. The cross section is determined to be 89 $\pm$ 4 (stat)
$\pm$ 12 (syst) pb, consistent with the standard model.
