On 31 August 2019, an interstellar comet was discovered as it passed through the Solar System (2I/Borisov). On the basis of initial imaging observations, 2I/Borisov seemed to be similar to ordinary Solar System comets1,2-an unexpected characteristic given the multiple peculiarities of the only known previous interstellar visitor, 1I/'Oumuamua3-6. Spectroscopic investigations of 2I/Borisov identified the familiar cometary emissions from CN (refs. 7-9), C2 (ref. 10), O I (ref. 11), NH2 (ref. 12), OH (ref. 13), HCN (ref. 14) and CO (refs. 14,15), revealing a composition similar to that of carbon monoxide-rich Solar System comets. At temperatures greater than 700 kelvin, comets also show metallic vapours that are produced by the sublimation of metal-rich dust grains16. Observation of gaseous metals had until very recently17 been limited to bright sunskirting and sungrazing comets18-20 and giant star-plunging exocomets21. Here we report spectroscopic observations of atomic nickel vapour in the cold coma of 2I/Borisov at a heliocentric distance of 2.322 astronomical units-equivalent to an equilibrium temperature of 180 kelvin. Nickel in 2I/Borisov seems to originate from a short-lived nickel-containing molecule with a lifetime of [Formula: see text] seconds at 1 astronomical unit and is produced at a rate of 0.9 ± 0.3 × 1022 atoms per second, or 0.002 per cent relative to OH and 0.3 per cent relative to CN. The detection of gas-phase nickel in the coma of 2I/Borisov is in line with the recent identification of this atom-as well as iron-in the cold comae of Solar System comets17.