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