Résumé : A search for a light charged Higgs boson, originating from the decay of a topquark and subsequently decaying into a charm quark and a strange antiquark, is presented.The data used in the analysis correspond to an integrated luminosity of 19.7 fb−1recordedin proton-proton collisions at √s = 8 TeV by the CMS experiment at the LHC. The searchis performed in the process t¯t → W±bH∓b, where the W boson decays to a lepton (electron ¯or muon) and a neutrino. The decays lead to a final state comprising an isolated lepton,at least four jets and large missing transverse energy. No significant deviation is observedin the data with respect to the standard model predictions, and model-independent upperlimits are set on the branching fraction B(t → H+b), ranging from 1.2 to 6.5% for a chargedHiggs boson with mass between 90 and 160 GeV, under the assumption that B(H+ → c¯s)= 100%.