  We determine constraints on sterile neutrino warm dark matter through direct
detection experiments, taking XENON100, XENON1T and DARWIN as examples. If
keV-scale sterile neutrinos scatter inelastically with bound electrons of the
target material, an electron recoil signal is generated. This can be used to
set limits on the sterile neutrino mass and its mixing with the active sector.
While not competitive with astrophysical constraints from X-ray data, the
constraints are the first direct laboratory bounds on sterile neutrino warm
dark matter, and will be in some parts of parameter space the strongest limits
on keV-scale neutrinos.
