A magnetar giant flare in the nearby starburst galaxy M82

Magnetar giant flares are rare explosive events releasing up to 10⁴⁷ erg in gamma rays in less than 1 second from young neutron stars with magnetic fields up to 10^15-16 G (refs. ^1,2). Only three such flares have been seen from magnetars in our Galaxy^3,4 and in the Large Magellanic Cloud⁵ in roughly 50 years. This small sample can be enlarged by the discovery of extragalactic events, as for a fraction of a second giant flares reach luminosities above 10⁴⁶ erg s^-1, which makes them visible up to a few tens of megaparsecs. However, at these distances they are difficult to distinguish from short gamma-ray bursts (GRBs); much more distant and energetic (10^50-53 erg) events, originating in compact binary mergers⁶. A few short GRBs have been proposed^7-11, with different amounts of confidence, as candidate giant magnetar flares in nearby galaxies. Here we report observations of GRB 231115A, positionally coincident with the starburst galaxy M82 (ref. ¹²). Its spectral properties, along with the length of the burst, the limits on its X-ray and optical counterparts obtained within a few hours, and the lack of a gravitational wave signal, unambiguously qualify this burst as a giant flare from a magnetar in M82.