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Half of all the elements in the universe heavier than iron were created byrapid neutron capture. The theory for this astrophysical `$r$-process' wasworked out six decades ago and requires an enormous neutron flux to make thebulk of these elements. Where this happens is still debated. A key piece ofmissing evidence is the identification of freshly-synthesised $r$-processelements in an astrophysical site. Current models and circumstantial evidencepoint to neutron star mergers as a probable $r$-process site, with theoptical/infrared `kilonova' emerging in the days after the merger a likelyplace to detect the spectral signatures of newly-created neutron-captureelements. The kilonova, AT2017gfo, emerging from thegravitational-wave--discovered neutron star merger, GW170817, was the firstkilonova where detailed spectra were recorded. When these spectra were firstreported it was argued that they were broadly consonant with an outflow ofradioactive heavy elements, however, there was no robust identification of anyelement. Here we report the identification of the neutron-capture elementstrontium in a re-analysis of these spectra. The detection of a neutron-captureelement associated with the collision of two extreme-density stars establishesthe origin of $r$-process elements in neutron star mergers, and demonstratesthat neutron stars contain neutron-rich matter.

Identification of strontium in the merger of two neutron stars