This study is focused on the genus Euphorbia L. in a part of northeast Iran, viz the three Khorassan provinces. Since there are many taxa of Euphorbia in Iran which are used in different industries and have significant effects on human and non-human life it is important to revise their taxonomy. With about 90 species, following Turkey with 91 species, Iran is the second richest country for Euphorbia in Asia. Of these, 30 species are distributed in the Khorassan provinces. This is the first comprehensive work on the genus in this region. According to ‘Flora Iranica’, there are 17 species of Euphorbia in northeast Iran, while according to our results, there are 30 species of Euphorbia in the Khorassan provinces alone. In addition to various new taxonomic and biogeographic results, a new species, viz E. chamanbidensis, is described. Euphorbia chamanbidensis is closely related to E. aucheri, but seed micro-morphological characters differentiate them. Two identification keys to the Euphorbia species of the studied area are provided, one based on macro-morphological characters and another based on seed micro-morphological characters. Phytogeographic analysis and distribution maps for all species are also presented.

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