Herbs, perennial, rhizomatous, 3–7.5 dm. Rhizomes 2–3 mm diam., not swollen. Culms erect, terete, 2–4 mm diam., smooth or rugulose. Cataphylls 1–2, pink to straw-colored, apex acute. Leaves: basal 1–2, cauline 1–2; auricles 1–4.9 mm, , apex rounded, membranaceous; blade green to straw-colored, terete, 20–30 cm × 1.5–4 mm, rugulose or smooth. Inflorescences terminal panicles of 25–66 heads, 7–13 cm, branches spreading; primary bract erect; heads 6–10-flowered, hemispheric to obovoid, 5–10 mm diam. Flowers: tepals straw-colored to brown, lanceolate, apex acuminate; outer tepals (2–)2.5–3.4 mm; inner tepals (2–)2.6–3.6 mm; stamens 6, anthers 1.5–2 times filament length. Capsules exserted, chestnut brown, 1-locular, subuloidsubulate, (2.4–)3–3.9 mm, apex tapering to subulate tip, valves separating at dehiscence. Seeds obovoid, 0.3–0.4 mm, not tailed. 2n = 40.
Phenology: Fruiting early summer–late fall.
Habitat: Montane meadows, riverbeds, canyons, aroyos
Elevation: 100–1600 m
Juncus dubius has rugulose stems and leaves throughout most of its range, but on the periphery (in Mariposa, San Diego, and Sonoma counties, California) the plants are smooth.