A search for lepton-flavour-violating decays of the Higgs boson into a tau lepton and either an electron or a muon is presented. The analysis uses data from proton-proton collisions at the Large Hadron Collider at $\sqrt{s}$ = 13 TeV, collected by the ATLAS detector and corresponding to an integrated luminosity of 36.1 $\text{fb}^{-1}$. The uncertainties in the results are dominated by the systematic error related to the fake-tau background calculation, which involved a data-driven technique. No significant excess of events is found over the Standard Model expectation and upper limits at 95% CL are placed in the branching ratios Br($H \to e\tau$) and Br($H \to \mu \tau$) of 0.47% and 0.28%, respectively. These limits are a big improvement with respect to previous ATLAS results, from Run 1.