This Letter presents a search for high mass e+e− or μ+μ−resonances in pp collisions at √s=7 TeV at the LHC. The data were recorded by the ATLAS experiment during 2010 and correspond to a total integrated luminosity of ∼40 pb−1. No statistically significant excess above the Standard Model expectation is observed in the search region of dilepton invariant mass above 110 GeV. Upper limits at the 95% confidence level are set on the cross section times branching ratio of Z′ resonances decaying to dielectrons and dimuons as a function of the resonance mass. A lower mass limit of 1.048 TeV on the Sequential Standard Model Z′ boson is derived, as well as mass limits on Z⁎ and E6-motivated Z′ models.