Southern Yellow Flax is a perennial, glabrous herb 2-7 dm tall. It has alternate (except sometimes for the lowermost 1 or 2 nodes), entire, and lance-shaped leaves up to 2.5 cm long, the upper ones pointed at the tips. The flowers are borne on stiffly-ascending branches. They have light yellow petals 4-8mm wide that are fused at the base. The inner sepals have glandular bases. The fruit are small (2 mm) dry, rounded capsules, commonly purple above (Rhoads and Klein 2002, Gleason and Cronquist 1991).

This species is best identified when fruiting.

Linum sulcatum, L. virginianum, L. striatum, and L. intercursum are the other yellow-flowered flax species found in New York. Linum sulcatum is an annual species, and its leaves have dark, stipular glands at the leaf bases (Voss 1985). L. virginianum and L. striatum differ from L. medium by having opposite leaves at several of the lower nodes, more spreading infloresences, and inner sepals which are not glandular.

Linum intercursum is similar in habit, stature and foliage to L. medium, but the fruit is ovoid with the carpels pointed at the top, and usually yellow, while L. medium is rounded with flat-topped carpels, and usually purple (Rhoads and Block 2000, Voss 1985, Gleason and Cronquist 1991).