Here we describe Allium telmatum Bogdanović, Brullo, Giusso & Salmeri, a new species of Allium sect. Codonoprasum from North Dalmatia (Croatia). Its chromosome number (2n = 32), karyotype, leaf anatomy, ecology and taxonomical relationships are examined. Several features, such as phenology (flowering in autumn), occurrence in coastal salt marshes, tetraploid chromosome number, and morphology, indicate that it is most closely related to the Tyrrhenian species Allium savii. On the basis of our herbarium survey, we present here a distribution map of the autumn-flowering species of Allium sect. Codonoprasum in the Mediterranean area.