Trematolobelia singularis, a short-lived perennial member of the bellflower family (Campanulaceae), is an unbranched shrub with stems 0.6 to 1.5 m (2 to 5 ft) long. Leaves on the upper surface are dark green, and pale green on the lower surface, linear-elliptic shaped and 10 to 18 cm (4 to 7 in) long. Inflorescences are erect, unbranched, and 30 to 42 cm (11.8 to 15.8 in) long. Capsules are subglobose and 9 to 12 cm (3.5 to 4.7 in) diameter. Fruit and seeds are unknown. This species differs from others of this endemic Hawaiian genus by the unbranched, erect flowering stalk.