The observation of Higgs boson production in association with a top quark pair (ttH), based on the analysis of proton–proton collision data corresponding to integrated luminosities of up to 79.8 fb − 1 at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. The observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations using Run II data and 6.3σ (5.1σ exp.) by combining Run 2 and Run 1 data sets.