Warm dark matter (WDM) may resolve the possible conflict between observed galaxy halos and the halos produced in cold dark matter simulations. Here, we present an extension of the minimal supersymmetric standard model to include WDM by adding a gauge singlet fermion, χ̄, with a portal-like coupling to the minimal supersymmetric standard model Higgs doublets. This model has the property that the dark matter is necessarily warm. In the case where Mχ̄ is mainly due to electroweak symmetry breaking, the χ̄ mass is completely determined by its relic density and the reheating temperature TR. For 102GeV TR 105GeV, the range allowed by χ̄ production via thermal Higgs annihilation, the χ̄ mass is in the range 0.3-4 keV, precisely the range required for WDM. The primordial phase-space density Q can directly account for that observed in dwarf spheroidal galaxies, Q 5×106(eV/cm3)/(km/s)3, when the reheating temperature is in the range TR 10-100TeV, in which case Mχ̄ 0.45keV. The free-streaming length is in the range 0.3-4 Mpc, which can be small enough to alleviate the problems of overproduction of galaxy substructure and low angular momentum of cold dark matter simulations. © 2009 The American Physical Society.