A bstract The cross section of top quark pair production is measured in the $$ \mathrm{t}\overline{\mathrm{t}}\to \left(\ell {\nu}_{\ell}\right)\left({\uptau}_{\mathrm{h}}{\nu}_{\uptau}\right)\mathrm{b}\overline{\mathrm{b}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>t</mml:mi> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> <mml:mo>→</mml:mo> <mml:mfenced> <mml:mrow> <mml:mi>ℓ</mml:mi> <mml:msub> <mml:mi>ν</mml:mi> <mml:mi>ℓ</mml:mi> </mml:msub> </mml:mrow> </mml:mfenced> <mml:mfenced> <mml:mrow> <mml:msub> <mml:mi>τ</mml:mi> <mml:mi>h</mml:mi> </mml:msub> <mml:msub> <mml:mi>ν</mml:mi> <mml:mi>τ</mml:mi> </mml:msub> </mml:mrow> </mml:mfenced> <mml:mi>b</mml:mi> <mml:mover> <mml:mi>b</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> final state, where τ h refers to the hadronic decays of the τ lepton, and ℓ is either an electron or a muon. The data sample corresponds to an integrated luminosity of 35.9 fb − 1 collected in proton-proton collisions at $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> = 13 TeV with the CMS detector. The measured cross section is $$ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>σ</mml:mi> <mml:mrow> <mml:mi>t</mml:mi> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:mrow> </mml:msub> </mml:math> = 781 ± 7 (stat) ± 62 (syst) ± 20 (lumi) pb, and the ratio of the partial width Γ(t → τ ν τ b) to the total decay width of the top quark is measured to be 0 . 1050 ± 0 . 0009 (stat) ± 0 . 0071 (syst). This is the first measurement of the $$ \mathrm{t}\overline{\mathrm{t}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>t</mml:mi> <mml:mover> <mml:mi>t</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> production cross section in proton-proton collisions at $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> = 13 TeV that explicitly includes τ leptons. The ratio of the cross sections in the ℓτ h and ℓℓ final states yields a value $$ {R}_{\ell {\uptau}_{\mathrm{h}}/\mathcal{ll}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mi>ℓ</mml:mi> <mml:msub> <mml:mi>τ</mml:mi> <mml:mi>h</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:mi>ll</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 0 . 973 ± 0 . 009 (stat) ± 0 . 066 (syst), consistent with lepton universality.