  We study the phenomenology of superheavy decaying dark matter with mass
around $10^{10}$ GeV which can arise in the low-energy limit of string
compactifications. Generic features of string theory setups (such as high scale
supersymmetry breaking and epochs of early matter domination driven by string
moduli) can accommodate superheavy dark matter with the correct relic
abundance. In addition, stringy instantons induce tiny $R$-parity violating
couplings which make dark matter unstable with a lifetime well above the age of
the Universe. Adopting a model-independent approach, we compute the flux and
spectrum of high-energy gamma rays and neutrinos from three-body decays of
superheavy dark matter and constrain its mass-lifetime plane with current
observations and future experiments. We show that these bounds have only a mild
dependence on the exact nature of neutralino dark matter and its decay
channels. Applying these constraints to an explicit string model sets an upper
bound of ${\cal O}(0.1)$ on the string coupling, ensuring that the effective
field theory is in the perturbative regime.
