Tetramolopium arenariurn is in the aster family (Asteraceae), and is an erect tufted shrub 0.8 to 1.3 m (2.6 to 4.3 ft) tall, is covered with tiny glands and straight hairs. The alternate, toothless or shallowly toothed leaves are more or less lance-shaped, 15 to 37 mm (0.6 to 1.5 in) long, and 3 to 9 mm (0.1 to 0.4 in) wide. Five to 11 heads (dense flower clusters) are grouped at the end of each stem. Each head comprises a bell-shaped structure of 20 to 34 bracts 2.5 to 5 mm (0.1 to 0.2 in) high and 4 to 9 mm (0.2 to 0.4 in) in diameter beneath the flowers; a single series of 22 to 45 white, male ray florets 1.3 to 2.2 mm (0.05 to 0.09 in) long; and 4 to 9 bisexual disk florets with maroon petals 3.1 to 4.4 mm (0.12 to 0.17 in) long. Fruits are compressed achenes 1.5 to 3 mm (0.06 to 0.1 in) long and 0.5 to 0.8 mm (0.02 to 0.03 in) wide. This species is distinguished from others of the genus by its erect habit; the presence and types of glands and hairs on the plant; the fewer heads per flower cluster; the larger, male ray florets; the fewer, bisexual, maroon-petalled disk florets; and the wider achenes. Lowrey’s taxonomic arrangement, which recognizes three infraspecific taxa, was accepted in Wagner et al. (1999): T. arenarium subsp. arenarium var. arenarium; T. arenarium subsp. arenarium var. confertum; and T. arenarium subsp. laxum. Tetramolopium arenarium subsp. arenarium var. confertum and T. arenarium subsp. laxum have not been observed since the late 1800s and are considered to be extinct.