Abstract: Glen Canyon National Recreation Area (NRA) is located in the Canyonlands district of the Colorado Plateau in southeastern Utah and northern Arizona. Glen Canyon NRA encompasses 1,254,117 acres of warm-temperate desert shrub, salt desert shrub, sand desert shrub, pinyon-juniper woodlands, riparian, and hanging garden vegetation communities. A total of 780 taxa of vascular plants, representing 90 families, are vouchered from within the park boundaries; 129 of these taxa are additions to the flora of Glen Canyon NRA. An additional 75 previously reported, but unvouchered, taxa were also identified in this study. Glen Canyon NRA is home to a number of rare and/or endemic species, including Cycladenia humilis Benth. var. jonesii (Eastw.) Welsh & Atwood, Astragalus preussii Gray var. cutleri Barneby, Phacelia indecora J.T. Howell, and Carex specuicola J.T. Howell. These species are of special interest to Glen Canyon NRA Natural Resource Division and Utah and Arizona Natural Heritage Programs, and are protected under the Endangered Species Act. About 40 species that occur in Glen Canyon NRA are endemic to the Colorado Plateau. Non-native plant taxa make up about 11% of the flora of Glen Canyon NRA. Several non-native taxa, including Brassica tournefortii Gouan, Cortaderia selloana (J.A. & J.H. Schultes) Aschers. & Graebn., and Saccharum ravennae (L.) L., pose threats to both Glen Canyon NRA and its downstream neighbor, Grand Canyon NP.