Trigonella yasujensis Ranjbar, Hajmoradi & Karamian sp. nova (Fabaceae) of T. sect. Ellipticae is described and illustrated from a single locality between Lordegan and Yasuj in the Zagros Mountains, Iran. It is closely related to T. elliptica but differs from it in a few morphological and anatomical characters. The meiotic chromosome number and behaviour were studied for T. yasujensis. It is a diploid species and possesses 2n = 2x = 16 chromosome number, consistent with the proposed base number x = 8. The general meiotic behaviour of the species was regular with normal bivalent pairing and chromosome segregation at meiosis. However, some meiotic abnormalities were observed including various degrees of sticky chromosomes with laggards, asynchronous nuclei, bridges and cytomixis.