A comparative study of the closely related species of the genus Gastridium (Poaceae, Tribe: Poeae, Subtribe: Agrostidinae), G. ventricosum, G. phleoides, and G. scabrum was assessed based on a critical examination of both fresh and dried specimens, including relevant material, and a morphometric analysis of herbarium specimens collected in Italy. The study aimed to provide new diagnostic tools to improve the taxonomy of the genus and confirm species delimitation. Thus, variation in floral morphology in the three species was further explored using seven quantitative and ten qualitative characters measured on 318 spikelets, both awned and unawned. Statistical methods, including principal components analysis (PCA) and non‐metric multidimensional scaling (NMDS), were used. Numerical analyses showed a general distinctness of spikelets and florets, consistent with the three studied species, and revealed among and within species variation patterns in between‐ both spikelet types. Accordingly, the most informative quantitative characters appeared to be the width and length of glumes and the length of awns, useful to distinguish G. phleoides with the substantially narrowest glumes and the significantly higher length of awns from the others, especially from G. scabrum, in which the width of glumes reaches the highest value. The most informative qualitative characters appeared to be the presence/absence of thick hairs or minute tubercles on the upper side of the lemma. The autonomy of the three studied species was confirmed. The taxonomic significance of the results was briefly discussed, and notes on the species dispersal were added. Some nomenclatural and taxonomic notes on G. scabrum, of which literature still showed evidence of a general unawareness, were given.