Abstract: If there exist unstable relics from the early universe, their decays could produce detectable fluxes of gamma rays and neutrinos. The decays of superheavy particles, m_χ ≳ 10^10 GeV, would produce an enhanced flux of ultra-high-energy neutrinos from large electroweak corrections, as well as muon and pion pair production which take place in the resulting electromagnetic cascades. These processes transfer energy from electromagnetic decay products into neutrinos, relaxing the constraints that can be derived from gamma-ray observations, and increasing the sensitivity of high-energy neutrino telescopes to superheavy particle decays. Taking both effects into account, I will present new constraints on long-lived superheavy relics from the IceCube Neutrino Observatory and the Fermi Large Area Telescope. IceCube-Gen2 and other next generation neutrino telescopes will provide unprecedented sensitivity to the decays of superheavy dark matter particles and other long-lived relics.