Juncus tenuis is a perennial herb; stems 1 to 6 dm long, 1 to 2 mm in diameter, loosely cespitose. Leaves basal, numerous and crowded, blades variable in length, 7 to 30 cm long, 1 to 1.3 mm wide, sheaths loose with wide hyaline margins prolonged into white scarious auricles up to 5 mm long. Flowers (3-) numerous, borne singly on 1 side of rachis, in relatively compact dichasial cymes 1 to 7 cm long, forming less than 1/3 of plant, bracts 2 to 3, foliaceous, usually far exceeding inflorescence, each flower subtended by 2 bracteoles, these hyaline, triangular-ovate; perianth parts greenish, lanceolate, 3 to 4.5 mm long, apex long-acuminate, outer ones slightly reflexed at apex, inner ones with wide hyaline margins below; stamens 6, 1/2 as long as perianth; anthers shorter than filaments. Capsules reddish brown, 1-celled, broadly ovoid, thin-walled, slightly beaked, 2/3 to rarely scarcely shorter than perianth. Seeds pale brown, oblong, 0.3 to 0.4 mm long, slightly apiculate at both ends (Wagner et al. 1999, in PIER 2007).

Juncus tenuis is found naturalised in Hawaii usually in wet sites along trails, disturbed places, and pastures at altitudes of 700 to 2000 meters (Wagner et al. 1999, in PIER 2007).

The Global Compendium of Weeds lists Juncus tenuis as an agricultural weed, casual alien, cultivation escape and environmental weed (GCW 2007).
J. tenuis has proven to be an ecological weed on Saint Helena where it alters valuable native habitats including breeding and nesting sites of the 'Critically Endangered (CR)' endemic wirebird (see Charadrius sanctaehelenae in IUCN Red List of Threatened Species) (Varnham 2006).