Three-horned bedstraw is a sprawling annual herb. The stems are up to 50 cm long, square in cross section and covered with downwardly pointing prickles, especially along the ribs. The leaves are long, narrow and flat and arranged in whorls; the first leaves are arranged in groups of four, with a pointed tip and a few hairs along the margins, the mature leaves are in whorls of 6 or 7, broader, with backwardly pointed prickles on the margins.
The flowers are small and white, appear in clusters of three and are carried on short branched stems arising from the leaf whorl. Each flower forms a pair of ‘seeds’ or nutlets that separate when mature.
Spread is by seed. Seed can be spread as a contaminant of agricultural products.