A bstract The production of a prompt photon in association with a Z boson is studied in proton-proton collisions at a centre-of-mass energy $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> = 13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb − 1 collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process pp → ℓ + ℓ − γ + X ( ℓ = e, μ ) is measured within a fiducial phase-space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of 2.9% is achieved for the fiducial cross-section. Differential cross-sections are measured as a function of each of six kinematic variables characterising the ℓ + ℓ − γ system. The data are compared with theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered.