Juncus brachyphyllus is a plant of floodplains, prairie remnants, and glades in the Midwest, from Tennessee to Texas, north to Illinois and Nebraska. Disjunct plants called J. brachyphyllus from Idaho to California are described here as a new species, J. trilocularis. They are distinguished by smoother stems and longer styles, and frequently have longer anthers. Rare putative hybrids between J. confusus and J. trilocularis are discussed from Lassen County, California and Kittitas County, Washington. Another putative hybrid, between J. occidentalis and J. trilocularis, is reported from Butte County, California. The new species is assigned to Juncus section Steirochloa Griseb., and a sectional key is provided for western North America.