This paper presents a direct measurement of the decay width of the top quark using tt̅ events in the lepton+jets final state. The data sample was collected by the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of 8 TeV and corresponds to an integrated luminosity of 20.2 fb⁻¹. The decay width of the top quark is measured using a template fit to distributions of kinematic observables associated with the hadronically and semileptonically decaying top quarks. The result, Γ_t = 1.76 ± 0.33 (stat.) _{-0.68} ^{+0.70} (syst.) GeV for a top-quark mass of 172.5 GeV, is consistent with the prediction of the Standard Model.