Grex or GREX may refer to:

  • Grex (biology), a multicellular aggregate of amoeba of the phyla Acrasiomycota or Dictyosteliomycota
  • Grex (horticulture), (pl. greges) a kind of group used in horticultural nomenclature applied to the progeny of an artificial cross from specified parents
  • Formerly used to mean species aggregate
  • Georgetown Rail Equipment Company, a provider of railway maintenance equipment and related services based in Georgetown, Texas