The cross-section for the production of a single top quark in association with a W boson in proton-proton collisions at s = 8 T e V $$ \sqrt{s}=8\mathrm{T}\mathrm{e}\mathrm{V} $$ is measured. The dataset corresponds to an integrated luminosity of 20.3 fb −1 , collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN. Events containing two leptons and one central b -jet are selected. The W t signal is separated from the backgrounds using boosted decision trees, each of which combines a number of discriminating variables into one classifier. Production of W t events is observed with a significance of 7 . 7 σ . The cross-section is extracted in a profile likelihood fit to the classifier output distributions. The W t cross-section, inclusive of decay modes, is measured to be 23.0 ± 1.3(stat.) − 3.5 + 3.2 (syst.)±1 . 1(lumi.) pb. The measured cross-section is used to extract a value for the CKM matrix element | V tb | of 1 . 01 ± 0 . 10 and a lower limit of 0.80 at the 95% confidence level. The cross-section for the production of a top quark and a W boson is also measured in a fiducial acceptance requiring two leptons with p T > 25 GeV and | η | < 2 . 5, one jet with p T > 20 GeV and | η | < 2 . 5, and E T miss > 20 GeV, including both W t and top-quark pair events as signal. The measured value of the fiducial cross-section is 0.85 ± 0.01(stat.) − 0.07 + 0.07 (syst.)±0 . 03(lumi.) pb.