  Ultrahigh-energy neutrinos (UHE$\nu$s) can be used as a valuable probe of
superheavy dark matter above $\sim 10^9$ GeV, the latter being difficult to
probe with collider and direct detection experiments due to the feebly
interacting nature. Searching for radio emissions originating from the
interaction of UHE$\nu$s with the lunar regolith enables us to explore energies
beyond $10^{12}$ GeV, which astrophysical accelerators cannot achieve. Taking
into account the interaction of UHE$\nu$s with the cosmic neutrino background
and resulting standard neutrino cascades to calculate the neutrino flux on
Earth, we investigate sensitivities of such lunar radio observations to very
heavy dark matter. We show that the proposed ultra-long wavelength lunar radio
telescope, as well as the existing low-frequency array, can provide the most
stringent constraints on decaying or annihilating superheavy dark matter with
masses at $\gtrsim 10^{12}$ GeV. The limits are complementary to or even
stronger than those from other UHE$\nu$ detectors, such as the IceCube-Gen2
radio array and GRAND.
