Juncus marginatus a grass-like herbaceous, short-lived perennial growing from short rhizomes. Plants produce dense tufts or clumps growing 30-130 cm tall. The plants are sometimes rhizomatous. The leaf blades are flat. The flowers are grouped together into a terminal inflorescence called a glomerulus. Each flower has three stamens and three sharply acute sepals that are reddish brown in color. The plump and ribbed seeds are produced in a rounded and beakless capsule. The small yellow to light brown seeds are spindle shaped, and around 0.5mm in length and lack a tail but have sharp points on either end (apiculate). The diploid chromosome count is 38.