A search for pair production of heavy scalar leptoquarks (LQs), each decaying into a top quark and a τ lepton, is presented. The search considers final states with an electron or a muon, one or two τ leptons that decayed to hadrons, and additional jets. The data were collected in 2016 in proton–proton collisions at s√=13TeV with the CMS detector at the LHC, and correspond to an integrated luminosity of 35.9fb−1. No evidence for pair production of LQs is found. Assuming a branching fraction of unity for the decay LQ→tτ, upper limits on the production cross section are set as a function of LQ mass, excluding masses below 900GeV at 95% confidence level. These results provide the most stringent limits to date on the production of scalar LQs that decay to a top quark and a τ lepton.