Abstract
The Koktau Mountains, located within the Kalba Highlands of Eastern Kazakhstan, represent an important yet insufficiently studied center of plant diversity in the Kazakhstan Altai. Field surveys conducted between 2018 and 2025 yielded more than 2700 herbarium specimens and were complemented by a revision of collections deposited in the ABG, AA, LE, and MW herbaria. Taxonomic, ecological, and incidence-based comparative analyses were used to characterize the flora and evaluate its relationships with neighboring regions. We recorded 817 accepted vascular plant taxa at species and infraspecific ranks, belonging to 356 genera and 79 families. The flora combines boreal, Mediterranean, and Turanian elements, reflecting the transitional position of the Koktau Mountains between the Altai mountain systems and the steppes of Eastern Kazakhstan. Twenty-seven rare and threatened taxa were identified, including 16 species listed in the Red Data Book of Kazakhstan and two species endemic to Kazakhstan, one of which, Elymus sibinicus, is currently known only from the Koktau Mountains. Floristic similarity was greatest with the Bukhtarma Mountains and Ulba Range. Because the compared inventories differ in area and survey history, these incidence-based relationships are interpreted descriptively. Together with the low number of endemic taxa, these relationships are consistent with the interpretation of the Koktau Mountains as a transitional flora influenced by regional floristic exchange.