We compute the third-order correction to electromagnetic
S-wave quarkonium production and annihilation rates due to the emission and
absorption of an ultrasoft gluon. Our result completes the analysis of the
non-relativistic quarkonium bound-state dynamics in the
next-to-next-to-next-to-leading order. The impact of the ultrasoft correction
on the Upsilon(1S) leptonic width and the top quark-antiquark threshold
production cross section is estimated.Comment: 10 page