Background and aims -The present study is focused on the cytogenetic and morphological criteria allowing to distinguish a new taxon from Onobrychis sect. Heliobrychis. This section is the largest section in O. subg. Sisyrosema represented with 21 species in Iran. The new species belongs to the O. subsect. Boissierianae characterized by perennial plants with well-developed stems and O. andalanica group with uniformely yellow corolla. Methods -The morphological features and meiotic chromosome number and behaviour were studied in O. avajensis Ranjbar. Key results -The novelty Onobrychis avajensis Ranjbar, endemic to Iran, is described and illustrated from two collections from a single locality between Avaj and Abgram in Qazvin Province in the west Zagros. It is closely related to O. andalanica Bornm. but differs from it in a few morphological characters. In addition, meiotic chromosome number and behaviour were studied in O. avajensis. This report is the first cytogenetic analysis of this taxon. O. avajensis is a diploid plant and possesses 2n = 2x = 16 chromosomes, consistent with the proposed base number of x = 8. The general meiotic behaviour of the species was regular, with bivalent pairing and normal chromosome segregation at meiosis. Meiotic abnormalities were observed included a varying degree of sticky chromosomes with laggards, precocious division of centromeres in metaphase I, bridges in anaphase I and multipolar cells in telophase II. Conclusion -Morphological features support the separation of Onobrychis avajensis Ranjbar as a new species.