  We investigate a novel reheating scenario proceeding through $s$-channel
inflaton annihilation, mediated by a massive scalar. If the inflaton $\phi$
oscillates around the minimum of a monomial potential $\propto \phi^{n}$, we
reveal the emergence of resonance phenomena originating from the dynamic
evolution of the inflaton mass for $n>2$. Consequently, a resonance appears in
both the radiation and the temperature evolution during the reheating process.
By solving the coupled Boltzmann equations, we present solutions for radiation
and temperature. We find non-trivial temperature characteristics during
reheating, depending on the value of $n$ and the masses of the inflaton and
mediator. Some phenomenological aspects of the model are explored. As a
concrete example, we show that the same mediator participates in the genesis of
dark matter, modifying the standard freeze-in dynamics. In addition, we
demonstrate that the resonant reheating scenario could be tested by
next-generation low- and high-frequency gravitational wave detectors.
