A search for new charged massive gauge bosons, called W’, decaying to tb, is performed with the ATLAS detector in the decay channel leading to final states with an electron or muon, 2 or 3 jets, and missing transverse momentum. This search uses a dataset corresponding to an integrated luminosity of 36.1 fb-1 of pp collisions produced at the LHC and collected during 2015 and 2016. The data is found to be consistent with the Standard Model expectation. Therefore limits are set on the W ′ → tb cross section times branching ratio and on the W ′ boson effective couplings as a function of the W ′ boson mass.