A bstract A search for a new massive charged gauge boson, W ′, is performed with the ATLAS detector at the LHC. The dataset used in this analysis was collected from proton-proton collisions at a centre-of-mass energy of $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> = 13 TeV, and corresponds to an integrated luminosity of 139 fb − 1 . The reconstructed tb invariant mass is used to search for a W ′ boson decaying into a top quark and a bottom quark. The result is interpreted in terms of a W ′ boson with purely right-handed or left-handed chirality in a mass range of 0.5–6 TeV. Different values for the coupling of the W ′ boson to the top and bottom quarks are considered, taking into account interference with single-top-quark production in the s -channel. No significant deviation from the background prediction is observed. The results are expressed as upper limits on the W ′ → tb production cross-section times branching ratio as a function of the W ′-boson mass and in the plane of the coupling vs the W ′-boson mass.