Magnetars are the most highly-magnetized neutron stars in the cosmos (B ⇠ 10 13 15 G). Giant flares from magnetars are rare, short-duration (about 0.1 s) bursts of hard X-rays and soft g rays 1, 2. We report here the discovery of GeV emission from a magnetar giant flare (MGF) on 15 April, 2020 3-5. The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected GeV g rays from 19 s until 284 s after the initial detection of a signal in the MeV band. Our analysis shows that these g rays are spatially associated with the nearby (3.5 Mpc) Sculptor galaxy and are unlikely to originate from a cosmological gray burst. Thus, we infer that the g rays originated with the MGF in Sculptor. We suggest that the GeV signal is generated by an ultra-relativistic outflow that first radiates the prompt MeV-band photons, and then deposits its energy far from the stellar magnetosphere. After a propagation delay, the outflow interacts with environmental gas, produces shock waves that accelerate electrons to very high energies and these then emit GeV g rays as optically thin synchrotron radiation. On 15 Apr 2020, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 200415A 4 initially classified as a short (duration<2 seconds) Gamma-ray Burst (SGRB). The Interplanetary Network of gray detectors (IPN 1) reduced the uncertainty on the GBM position to 20 sq. arcmin suggesting that the GRB originated from the nearby Sculptor galaxy 3 , located at a distance of ⇠3.5 Mpc 6. This, with the resemblance of the GBM sub-MeV light curve 7 to the extragalactic Soft Gamma Repeater (SGR) giant flare candidates GRB 051103 3, 8 and GRB 070201 9 , and the detection of quasi-periodic oscillations (QPOs) by the Atmosphere-Space Interaction Monitor (ASIM) 5 , led to the identification of GRB 200415A as a Magnetar Giant Flare (MGF) in Sculptor. GRB 200415A was 43 from the LAT boresight at the GBM trigger time T 0 (08:48:05.563746 UTC) and remained well within the LAT field of view (FOV) until 500 seconds after T 0. Three g rays were detected by the LAT, allowing the localization of GRB 200415A at high energies (>100 MeV): this represents the first detection of high-energy gamma-ray emission from an MGF, and suggests that magnetars can power the relativistic outflows observed in some SGRBs. To study the localization of the gray signal observed by the LAT we perform a likelihood analysis and compute a test statistic (TS) for the presence of the source at different positions. The best position is obtained from the maximum of the TS (TS max = 29, corresponding to a detection significance close to 5 s , see the Method section). Then, the variation of the TS around this position provides the map of localization contours shown in Figure 1. The iso-contours in red encompass localization probabilities of 68% and 90%. Four galaxies (IC 1576, IC 1578, IC 1582 and NGC 253) from the NGC 2000 catalog 10 are located within a circular region of radius r 99 , whose area is equivalent to the 99% c.l., and which is centered on the maximum of...