Within the general Uð1Þ scenario, we demonstrate that the ultrahigh-energy neutrinos recently detected by KM3NeT could originate from a decaying right-handed neutrino dark matter, with a mass of 440 PeV. Considering dark matter production via freeze-in, we delineate the parameter space that satisfies the observed relic abundance and also lies within the reach of multiple gravitational wave detectors. Our study provides a testable new physics scenario, enabled by multimessenger astronomy.