  The first observation of the $\mathrm{t\bar{t}}$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 TeV
collected by the CMS detector at the LHC, corresponding to an integrated
luminosity of 137 fb$^{-1}$. The cross section times branching fraction of the
$\mathrm{t\bar{t}}$H process is measured to be
$\sigma_{\mathrm{t\bar{t}H}}\mathcal{B}_{\gamma\gamma} =$ 1.56
$^{+0.34}_{-0.32}$ fb, which is compatible with the standard model prediction
of 1.13 $^{+0.08}_{-0.11}$ fb. The fractional contribution of the CP-odd
component is measured to be $f^{\mathrm{Htt}}_\mathrm{CP} =$ 0.00 $\pm$ 0.33.
