A search for the production of heavy resonances decaying into top quark-antiquark pairs is presented. The analysis is performed in the lepton+jets and fully hadronic channels using data collected in proton-proton collisions at s=13 TeV using the CMS detector at the LHC, corresponding to an integrated luminosity of 2.6 fb−1. The selection is optimized for massive resonances, where the top quarks have large Lorentz boosts. No evidence for resonant t t ¯ production is found in the data, and upper limits on the production cross section of heavy resonances are set. The exclusion limits for resonances with masses above 2 TeV are significantly improved compared to those of previous analyses at s=8 TeV.[Figure not available: see fulltext.].