  In the next decade, the proposed Line Emission Mapper (LEM) telescope concept
is poised to revolutionize Galactic and extragalactic X-ray sensitivity. The
instruments aboard LEM feature unprecedented eV scale energy resolution and an
effective area of 1600 cm$^2$ at 0.5 keV. Such features are ideally suited to
explore decaying dark matter candidates that predict X-ray signals, including
axion-like particles and sterile neutrinos. We present the first forecast of
LEM sensitivity to dark matter decays and find sensitivity to lifetimes beyond
$\sim 10^{32}$ s in the keV range, surpassing current limits by several orders
of magnitude. Notably, our results show that LEM will be the first ever
instrument to probe such long dark matter lifetimes in any mass range for any
decay channel.
