Neutrino self-interactions ($\nu \mathrm{SI}$) mediated by light bosonic particles can produce characteristic spectral dips in astrophysical neutrino fluxes, thereby altering the expected energy spectrum. The high-energy astrophysical neutrino spectrum has been extensively used to probe $\nu \mathrm{SI}$ models through these distinctive features. The recent detection of the ultrahigh-energy event KM3-230213A presents a new opportunity to explore $\nu \mathrm{SI}$ phenomenology at extreme energies. In this work, we investigate two implications of this observation, assuming the event originates from a diffuse power-law spectrum. First, we find that $\nu \mathrm{SI}$-induced spectral distortions can mildly alleviate the tension between the KM3-230213A detection and the previous nonobservation of PeV-scale neutrinos in IceCube data. Second, we derive the strongest constraints on the $\tau$-flavored $\nu \mathrm{SI}$ coupling strength for mediator masses around 100 MeV. Our analysis shows that neutrino telescopes can surpass existing collider bounds in this mass range. In the near future, IceCube-Gen2 is expected to significantly enhance $\nu \mathrm{SI}$ sensitivity, including regions relevant to alleviating the Hubble and neutrino mass tensions.

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• Extrasolar neutrino astronomy
• Particle astrophysics
• Particle interactions
• Phenomenology
• Neutrinos