Juncus marginatus is native in eastern North America, west to Arizona. Recent California treatments considered it a native species, and rare. Its collection history in Oregon and California was examined to determine if those populations are native or introduced. The earliest Oregon records are 1991 from disturbed sites. The earliest Calfornia records are from disturbed mining sites in 1965 and 1971. At least one California population is associated with exotic cranberry, Vaccinium macrocarpon, a documented vector for transport of propagules of wetland species native to eastern North America. Eight other Juncus species native to eastern North America are naturalized in the Pacific States, lending support to introduced status for J. marginatus in California. Subsequent discoveries of J. marginatus in Oregon and California suggest the species is expanding its adventive range into less disturbed plant communities.