  Warm dark matter (WDM) might more easily account for small scale clustering
measurements than the heavier particles typically invoked in Lambda cold dark
matter (LCDM) cosmologies. In this paper, we consider a Lambda WDM cosmology in
which sterile neutrinos nu_s, with a mass m_s of roughly 1-100 keV, are the
dark matter. We use the diffuse X-ray spectrum (total minus resolved point
source emission) of the Andromeda galaxy to constrain the rate of sterile
neutrino radiative decay: nu_s -> nu_{e,mu,tau} + gamma. Our findings demand
that m_s < 3.5 keV (95% C.L.) which is a significant improvement over the
previous (95% C.L.) limits inferred from the X-ray emission of nearby clusters,
m_s < 8.2 keV (Virgo A) and m_s < 6.3 keV (Virgo A + Coma).
