The genus Xanthoparmelia belongs to one of the largest and most species-rich foliose lichen families - Parmeliaceae. It occurs primarily in arid regions around the world. In the United States, Xanthoparmelia is extremely abundant in the southern Rocky Mountains and adjacent high plains of Colorado, where species play significant ecological roles. The present study emphasizes the examination of type material, protologues, study of new field collections as well as existing herbarium material, data from thin layer chromatography, and information on species distributions to delimit species of non-melanin-containing (i.e., yellow-green) members of Xanthoparmelia in Colorado, U.S.A. Using the University of Colorado Herbarium (COLO) collection in addition to collections made by the authors, a total of 18 species belonging to five different chemical groups are recognized as occurring in Colorado. Xanthoparmelia arseneana is placed in synonymy with X. novomexicana. In addition, two species are excluded from Colorado: X. taractica and X. hypopsila. A dichotomous key and a species treatment that includes type citations, morphological descriptions, chemical information, and geographical distributions is included. This treatment is the first to focus on this abundant yet taxonomically challenging lineage solely in Colorado.