  Neutron-star mergers were recently confirmed as sites of
rapid-neutron-capture (r-process) nucleosynthesis. However, in Galactic
chemical evolution models, neutron-star mergers alone cannot reproduce the
observed element abundance patterns of extremely metal-poor stars, which
indicates the existence of other sites of r-process nucleosynthesis. These
sites may be investigated by studying the element abundance patterns of
chemically primitive stars in the halo of the Milky Way, because these objects
retain the nucleosynthetic signatures of the earliest generation of stars. Here
we report the element abundance pattern of the extremely metal-poor star SMSS
J200322.54-114203.3. We observe a large enhancement in r-process elements, with
very low overall metallicity. The element abundance pattern is well matched by
the yields of a single 25-solar-mass magnetorotational hypernova. Such a
hypernova could produce not only the r-process elements, but also light
elements during stellar evolution, and iron-peak elements during explosive
nuclear burning. Hypernovae are often associated with long-duration gamma-ray
bursts in the nearby Universe. This connection indicates that similar
explosions of fast-spinning strongly magnetized stars occurred during the
earliest epochs of star formation in our Galaxy.
