  Half of all the elements in the universe heavier than iron were created by
rapid neutron capture. The theory for this astrophysical `$r$-process' was
worked out six decades ago and requires an enormous neutron flux to make the
bulk of these elements. Where this happens is still debated. A key piece of
missing evidence is the identification of freshly-synthesised $r$-process
elements in an astrophysical site. Current models and circumstantial evidence
point to neutron star mergers as a probable $r$-process site, with the
optical/infrared `kilonova' emerging in the days after the merger a likely
place to detect the spectral signatures of newly-created neutron-capture
elements. The kilonova, AT2017gfo, emerging from the
gravitational-wave--discovered neutron star merger, GW170817, was the first
kilonova where detailed spectra were recorded. When these spectra were first
reported it was argued that they were broadly consonant with an outflow of
radioactive heavy elements, however, there was no robust identification of any
element. Here we report the identification of the neutron-capture element
strontium in a re-analysis of these spectra. The detection of a neutron-capture
element associated with the collision of two extreme-density stars establishes
the origin of $r$-process elements in neutron star mergers, and demonstrates
that neutron stars contain neutron-rich matter.
