A search for a light charged Higgs boson decaying into cs is presented using data recorded in pp collisions at √s = 7 TeV. The analysis uses a data sample corresponding to an integrated luminosity of 35.3 pb-1 collected by the ATLAS detector at theLHC between June and November 2010. The search is based on the semi-leptonic tt channel searching for the process t → H+b where H+ → cs. The invariant mass distri- bution of the dijet system consistent with the hadronic decay of a W+ is used to search for a secondary bump from hadronic H+ decays. With no observation of the charged Higgs signal, 95% confidence level upper limits on the decay branching ratio of top quarks to charged Higgs bosons are set as a function of the charged Higgs boson mass.