  Gravitational waves have become an irreplaceable tool for exploring the
post-inflationary universe. Their cosmological and astrophysical origins have
been attracting numerous attention. In this Letter, we point out a novel source
of ultra-high frequency gravitational waves: the decay of particles produced
during the reheating era. We highlight the decay of the Higgs boson as a
representative case, showing how it yields a testable gravitational wave
spectrum by future observations.
