Plants 40-120 cm. Cauline leaves: largest at mid stems; blades usually 3-nerved, usually lanceolate to elliptic, rarely ovate, 12-25 cm, faces puberulent, hirsute, or scabrous. Heads usually borne singly, sometimes 2-3+, erect. Involucres turbinate to hemispheric or broader, 15-30 mm diam. Phyllaries lanceolate to lance-linear (subequal or outer larger, margins sometimes ciliate, faces ± cinereous-pubescent to sparsely puberulent). Paleae chartaceous, relatively firm. Ray florets 11-13(-21); laminae 15-30(-45) mm. Disc corollas yellow. Cypselae strongly compressed, narrowly obovate (margins ciliate, faces strigose); pappi of 2 ± subulate scales plus 2-4 shorter scales. 2n = 30.
Two infraspecific taxa within Helianthella uniflora may be distinguished; they have been named at varietal rank.
Variety douglasii has stems hirsute; involucres (15-)20-25(-30) mm diam.; outer phyllaries rarely elongated, margins ciliate, abaxial faces sparsely puberulent; ray laminae 30-40 mm; and 2n = 30. It grows in grasslands in the northern Rocky Mountains and on the east side of the Cascade Range (B.C.; Idaho, Oreg., Wash.) at 300-2500 m where it flowers May-Jul.
Variety uniflora has stems sparingly puberulent to scabrous; involucres 15-20(-25) mm diam.; outer phyllaries frequently elongated, margins not ciliate, abaxial faces uniformly cinereous-pubescent; ray laminae 20-30 mm; and 2n = 30. It grows in meadows, sagebrush scrublands, swales, aspen forests, rocky slopes, and spruce-fir forests of the Rocky Mountains, intermountain plateaus, and Great Basin ranges (Ariz., Colo., Idaho, Mont., Nev., Oreg., Utah, Wyo.) at 1500-3400 m, where it flowers (May-)Jun-Jul(-Aug).