Perennial herb with an upright, hairy stalk. Flowers clustered in leaf axils on very short stems, almost hidden by green calyx, tubular with 5 small lobes, reddish brown. Blooms May–July. Leaves opposite, the middle 3–5 pairs joined around the stem (perfoliate), broadly lanceolate, with smooth margins. Fruit berries that resemble little oranges, remaining on the stalks through fall.
Similar species: Two other horse gentians grow in Missouri. Yellow-flowered horse gentian (T. angustifolium), found in the Ozarks, has narrower leaves that are sessile or have winged stems (leaves are not perfoliate) and yellow flowers (sometimes orange to red). Red-fruited horse gentian (T. aurantiacum), scattered mostly in the eastern half of the state, is only weakly perfoliate, with leaves much narrower at the base, and has orange to red fruits. It can be hard to distinguish from T. perfoliatum.