The first observation of the (t\overline{t}H) process in a single Higgs boson decay channel with the full reconstruction of the final state ((H\to \gamma \gamma )) is presented, with a significance of 6.6 standard deviations ((\sigma )). The (CP) structure of Higgs boson couplings to fermions is measured, resulting in an exclusion of the pure (CP)-odd structure of the top Yukawa coupling at (3.2\sigma ). The measurements are based on a sample of proton-proton collisions at a center-of-mass energy (\sqrt{s}=13\text{}\text{}\mathrm{TeV}) collected by the CMS detector at the LHC, corresponding to an integrated luminosity of (137\text{}\text{}{\mathrm{fb}}^{-1}). The cross section times branching fraction of the (t\overline{t}H) process is measured to be ({\sigma }{t\overline{t}H}{B}{\gamma \gamma }={1.56}{-0.32}^{+0.34}\text{}\text{}\mathrm{fb}), which is compatible with the standard model prediction of ({1.13}{-0.11}^{+0.08}\text{}\text{}\mathrm{fb}). The fractional contribution of the (CP)-odd component is measured to be ({f}_{CP}^{Htt}=0.00±0.33).