Crassula tetragona is an evergreen shrub with narrow, almost needle-like foliage that grows into a sturdy, neatly shaped bush. It is an erect or spreading shrublet to 1 m, with a woody stem covered in brown bark. The leaves are in opposite pairs, lance-shaped with upward pointing tips. It bears masses of tiny, creamy white flowers in flat-topped clusters in late spring and summer. The flowers turn orange as they fade.

Six subspecies of Crassula tetragona are recognised:

subspecies acutifolia, a decumbent, carpet-forming shrublet up to 150 mm tall, leaves 5-15 x 1-2 mm, inflorescense irregularly branched in late summer, growing in low Karoo vegetation in the Western and Eastern Cape.

subspecies connivens, a stiffly upright shrublet with upward-pointing branches up to 1 m tall, leaves 18-27 x 1-2 mm, ascending, inflorescence round-topped with indisctinct peduncle in autumn, growing on north-facing sandstone ridges in the Little Karoo.

subspecies lignescens, an upright or scrambling shrublet to 800 mm with woody branches and flaking bark, leaves 12-20 x 2-3 mm, inflorescence round-topped, many-flowered, from midsummer to autumn, the most common subspecies growing in karroid scrub in the Northern, Western and Eastern Cape.

subspecies robusta, a robust, upright, shrubby succulent usually about 500 mm but up to 1 m, leaves 20-30 x 3-5 mm with blunt tips, inflorescence round-topped in late summer, grows in scrub in sheltered valleys in the Eastern Cape.

subspecies rudis, an upright, sparsely branched shrub to 250 mm tall, leaves 8-25 x 2-3 mm, inflorescence irregularly branched, from midsummer to autumn, growing under other shrubs on rocky slopes in succulent vegetation in the Northern and Western Cape.

subspecies tetragona, succulent shrubs, upright or prostrate with erect tips, 100-200 mm tall, leaves 10-25 x 2-3 mm, inflorescence a dense flat-topped cluster of tiny cream to white flowers on a long peduncle from midsummer to autumn, growing in dry vegetation in the Northern, Western and Eastern Cape provinces.