We present observations of the young supernova remnant (SNR) RXJ1713.7-3946 with the Fermi Large Area Telescope (LAT). We clearly detect a source positionally coincident with the SNR. The source is extended with a best-fit extension of 055 ± 004 matching the size of the non-thermal X-ray and TeV gamma-ray emission from the remnant. The positional coincidence and the matching extended emission allow us to identify the LAT source with SNR RXJ1713.7-3946. The spectrum of the source can be described by a very hard power law with a photon index of Γ = 1.5 0.1 that coincides in normalization with the steeper H.E.S.S.-detected gamma-ray spectrum at higher energies. The broadband gamma-ray emission is consistent with a leptonic origin as the dominant mechanism for the gamma-ray emission. © 2011. The American Astronomical Society. All rights reserved.