Ulmus okanaganensis is an extinct species of flowering plant in the family Ulmaceae related to the modern elms. The species is known from fossil leaves, flowers, and fruits found in the early Eocene deposits of northern Washington state, United States and similar aged formations in British Columbia, Canada. History and classification U. okanaganensis fossils have been identified from five locations in Western North America. The type description listed occurrences at the 49 million year old Klondike Mountain Formation near Republic, Washington plus the British Columbian sites of One Mile Creek near Princeton, British Columbia, the McAbee Fossil Beds east of Cache Creek, and in the Driftwood Canyon Provincial Park northeast of Smithers. In 2009 the species was reported from a fourth British Columbian site near Falkland. Ages for the Okanagan Highland locations are, in general, Early Eocene, with the sites that have current uranium-lead or argon–argon radiometric dates being of Ypresian age, while the undated sites or those given older dates being possibly slightly younger and Lutetian in age. Roland Brown examined fossils from the Klondike Mountain formation in a 1936 paper and placed some leaves into the extinct species Zelkova oregoniana along with fossils from the Latah Formation in Washington, which the Republic fossils were thought to belong to, and the John Day Formation in Oregon. Ulmaceous fruits from the Princeton area had been tentatively identified as a species of Chaetoptelea, a southern North American genus that is now included into Ulmus. The discovery of leaves attached to branchlets with fruits, and attached to branchlets attached to flowers at several fossil sites led to a restudy of the Okanagan highlands leaves and fruits. Fossils of leaves and fruits that had previously been identified as Zelkova and Chaetoptelea along with additional specimens were studied by paleobotanists Thomas Denk and Richard Dillhoff, with the type description for U. okanaganensis being published in a 2005 Canadian Journal of Botany article. They chose the specific name okanaganensis, in reference to the Okanagan Highlands, of Central British Columbia and northern Washington, where the species is a common paleofloral element of the Eocene lake deposits in the region. Based on phylogenetic analysis that was performed by Denk and Dillhoff, U. okanaganensis was placed as a member of Ulmus subgenus Ulmus, between U. section Microptelea and U. section Ulmus. Denk and Dillhoff noted that the results of the analysis also suggested the subgenus to be a