This dissertation presents the first observation of Higgs boson production in association with a top quark-antiquark pair in the diphoton decay channel, with a significance of 6.6 standard deviations. The measurement is performed with a dataset of 13 TeV proton-proton collisions recorded by the Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of 137 \fbinv.