Herbs, perennial, cespitose, 1.4--5.5(--7) dm. Culms erect, terete, 1--3 mm diam., smooth. Cataphylls 0--1, straw-colored to pink, apex acute. Leaves: basal 1--3, cauline 1--2; auricles 0.5--3 mm, apex rounded to truncate, scarious; blade terete, 1.5--25 cm x 0.5--2.5 mm. Inflorescences terminal panicles or racemes of 2--35 heads, 1--12 cm, branches erect; primary bract erect; heads 2--8-flowered, ellipsoid to narrowly obconic, 2--9 mm diam. Flowers: tepals green to light brown, lanceolate; outer tepals 2.3--3.1 mm, apex acuminate to rarely obtuse; inner tepals 2.5--3.2 mm, apex acuminate; stamens 3 (or 6), anthers 1/4--1/2 filament length. Capsules exserted, chestnut brown, imperfectly 3-locular, narrowly ellipsoid to prismatic, 3.2--4.8 mm, apex acute proximal to beak, valves separating at dehiscence. Seeds fusiform, 0.7--1.2 mm, tailed; body covered with whitish translucent veil. 2n = 80. Fruiting mid summer--fall. Generally in acidic or peaty moist sites, including emergent shorelines and aroundg hot springs; 100--2500 m; Alta., B.C., Man., N.B., Nfld. and Labr., N.S., Ont., P.E.I., Que., Sask.; Ariz., Colo., Conn., Maine, Md., Mass., Mich., Minn., Mont., N.H., N.Y., N.C., Oreg., Pa., R.I., Tenn., Utah, Vt., Va., W.Va., Wis., Wyo. Populations from about around hot springs in the west have been separated as Juncus tweedyi Rydberg, but no morphologic distinction appears to exist between J. tweedyi and J. brevicaudatus.