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Gastridium is a Mediterranean-paleotropical genus of the Poaceae family, native to Italy. Species number and diversity were imperfectly known until recent taxonomic updates on morphological and molecular basis that enhanced our knowledge of this taxon. The present contribution provides a complete key of the genus, encompassing the four currently known closely related species, G. lainzii, G. phleoides, G. scabrum, and G. ventricosum. The essential features of panicle, spikelets, and florets are specified and briefly discussed. Revisions of ancient and recent herbarium specimens provided three Italian distributional novelties for G. phleoides concerning Liguria, Campania, and Puglia and two for G. scabrum concerning Liguria and Basilicata. In contrast, the distributional ranges of G. scabrum and G. lainzii in the W Mediterranean region remain poorly known and await further investigations.