  The rare decay $H\to \gamma\gamma$ is a promising detection channel for an
intermediate mass Higgs boson. We compute its two-loop $O(\alpha^2 G_F m_t^2)$
correction in the standard model and find that the relative correction to the
decay rate runs between $0.7%$ and $0.5%$ for $M_H=80-150$ GeV. The analogous
correction to the amplitude for $gg\to H$ is recovered as a special case. The
generalization of our result to other models is also briefly indicated.
