Soft γ-ray repeaters exhibit bursting emission in hard X-rays and soft γ-rays. During the active phase, they emit random short (milliseconds to several seconds long), hard-X-ray bursts, with peak luminosities of 10 to 10 erg per second. Occasionally, a giant flare with an energy of around 10 to 10 erg is emitted . These phenomena are thought to arise from neutron stars with extremely high magnetic fields (10 to 10 gauss), called magnetars . A portion of the second-long initial pulse of a giant flare in some respects mimics short γ-ray bursts , which have recently been identified as resulting from the merger of two neutron stars accompanied by gravitational-wave emission . Two γ-ray bursts, GRB 051103 and GRB 070201, have been associated with giant flares . Here we report observations of the γ-ray burst GRB 200415A, which we localized to a 20-square-arcmin region of the starburst galaxy NGC 253, located about 3.5 million parsecs away. The burst had a sharp, millisecond-scale hard spectrum in the initial pulse, which was followed by steady fading and softening over 0.2 seconds. The energy released (roughly 1.3 × 10 erg) is similar to that of the superflare from the Galactic soft γ-ray repeater SGR 1806-20 (roughly 2.3 × 10 erg). We argue that GRB 200415A is a giant flare from a magnetar in NGC 253.