Gomphocarpus tomentosus is an erect herb with a woody base, growing to heights around 1,3 m. The plant material contains a milky latex, exuded when the skin is damaged. Young branches are densely woolly or tomentose. The specific epithet, tomentosa, is derived from this woolly covering, also found on the lower leaf surfaces.
The leaves are opposite, narrow and linear, up to 12 cm long. Being so narrow, there are many of them. They taper to their tips and have margins that are slightly rolled under. The leaves are generally carried erectly or ascending (iSpot; JSTOR; www.zimbabweflora.co.zw).