We report the detection of GeV γ -ray emission from the supernova remnant (SNR) Puppis A with the Fermi Gamma-Ray Space Telescope. Puppis A is among the faintest SNRs yet detected at GeV energies, with a luminosity of only 2.7×1034 (D/2.2 kpc)2 erg s−1 between 1 and 100 GeV. The γ -ray emission from the remnant is spatially extended, with a morphology matching that of the radio and X-ray emission, and is well described by a simple power law with an index of 2.1.We attempt to model the broadband spectral energy distribution (SED), from radio to γ -rays, using standard nonthermal emission mechanisms. To constrain the relativistic electron population we use 7 years ofWilkinsonMicrowave Anisotropy Probe data to extend the radio spectrum up to 93 GHz.Both leptonic- and hadronic-dominated models can reproduce the nonthermal SED, requiring a total content of cosmic-ray electrons and protons accelerated in Puppis A of at least WCR ≈ (1-5) × 1049 erg.