  We show that a thermal relic which decouples from the standard model (SM)
plasma while relativistic can be a viable dark matter (DM) candidate, if the
decoupling is followed by a period of entropy dilution that heats up the SM,
but not the dark sector. Such diluted hot relics can be as light as a keV,
while accounting for the entirety of the DM, and not conflicting with
cosmological and astrophysical measurements. The requisite dilution can be
achieved via decays of a heavy state that dominates the energy budget of the
universe in the early matter dominated era. The heavy state decays into the SM
particles, heats up the SM plasma, and dilutes the hidden sector. The
interaction required to equilibrate the two sectors in the early universe
places a bound on the maximum possible dilution as a function of the decoupling
temperature. As an example of diluted hot relic DM we consider a light Dirac
fermion with a heavy dark photon mediator. We present constraints on the model
from terrestrial experiments (current and future), astrophysics, and cosmology.
