A search for a Higgs boson produced in association with a pair of top quarks (t¯) and decaying into a pair of bottom quarks (H→ b¯ b) is presented. The analysis uses 4.7 fb −1 of pp collision data at √ s = 7 TeV collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The search is focused on the semileptonic decay mode of the t¯ t system which, together with the H → bdecay, typically results in a final state signature characterised by one high transverse momentum isolated electron or muon, high transverse missing momentum and six jets, of which four jets originate from b quarks. Events are classified in nine different topologies depending on their jet and b-tagged jet multiplicities in order to improve the sensitivity of the search. No significant excess of events above the background expectation is observed and 95% confidence- level upper limits on the production cross section times branching ratio, �(t¯ tH)× BR(H→ b¯), are derived for a Higgs boson with a mass between 110 and 140 GeV. For a Higgs boson with a mass of 125 GeV, an observed (expected) 95% confidence-level upper limit of 13.1 (10.5) times the Standard Model cross section is obtained.