Phyllostegia knudsenii, a member of the Lamiaceae (mint) family, is an erect, perennial herb or vine. The opposite leaves are limp, ovate, faintly pubescent, 11.5 to 18 cm (4.5 to 7 in) long, and 5.1 to 9 cm (2 to 3.5 in) wide. Flowers are borne in groups of two to four along a flower stalk 4 to 6.5 cm (1.6 to 2.6 in) long. The corolla is 6 to 8 mm (0.2 to 0.3 in) long. The fruits are four black fleshy nutlets in each flower and are 1.5 to 2 mm (0.06 to 0.8 in) long.