We report the first detection of > 100 MeV gamma rays associated with a behind-the-limb solar flare , which presents a unique opportunity to probe the underlying physics of high-energy flare emission and particle acceleration . On 2013 October 11 a GOES M1.5 class solar flare occurred \sim 9 ^ { \circ } .9 behind the solar limb as observed by STEREO -B . RHESSI observed hard X-ray emission above the limb , most likely from the flare loop-top , as the footpoints were occulted . Surprisingly , the Fermi Large Area Telescope ( LAT ) detected > 100 MeV gamma-rays for \sim 30 minutes with energies up to 3 GeV . The LAT emission centroid is consistent with the RHESSI hard X-ray source , but its uncertainty does not constrain the source to be located there . The gamma-ray spectra can be adequately described by bremsstrahlung radiation from relativistic electrons having a relatively hard power-law spectrum with a high-energy exponential cutoff , or by the decay of pions produced by accelerated protons and ions with an isotropic pitch-angle distribution and a power-law spectrum with a number index of \sim 3.8 . We show that high optical depths rule out the gamma rays originating from the flare site and a high-corona trap model requires very unusual conditions , so a scenario in which some of the particles accelerated by the CME shock travel to the visible side of the Sun to produce the observed gamma rays may be at work .