  Inflaton seeds non-thermal leptogenesis by pair producing right-handed
neutrinos in the seesaw model. We show that the inevitable graviton
bremsstrahlung associated with inflaton decay can be a unique probe of
non-thermal leptogenesis. The emitted gravitons contribute to a high-frequency
stochastic gravitational waves background with a characteristic fall-off below
the peak frequency. Besides leading to a lower bound on the frequency
($f\gtrsim 10^{11}$ Hz), the seesaw-perturbativity condition makes the
mechanism sensitive to the lightest neutrino mass. For an inflaton mass close
to the Planck scale, the gravitational waves contribute to sizeable dark
radiation, which is within the projected sensitivity limits of future
experiments such as CMB-S4 and CMB-HD.
