S. thermarum is an erect branched shrub, up to 1m in height with a stoloniferous base. The branches are dark green, up to 3 mm in diameter; the young stems are glandular, hairy becoming smooth with age. The leaves are also dark green with purplish tips, no petiole, not fragrant when crushed, oblanceolate to linear oblanceolate, 20-35 mm x 15 mm, serrated at the apex and entire at base.

The inflorescence is a short dense raceme 80-10 mm long (not branched); 3 to 4 verticillasters —  whorl of flowers apparently of one cluster, but composed of two opposite axillary cymes, 2-flowered, pale red to deep salmon corollas, 40-50 mm long, with red bracts. S. thermarum could be easily confused with its relative S. granitica, a species confined to an area south of Citrusdal and the Caledon District. But the latter is a smaller shrub, with a mauve-pink corolla, only 20 mm long and elongated racemes.

Ernst van Jaarsveld, who described the species, noticed that the galls on the stems are perhaps the result of a parasitic wasp. They appear as small growths of about 10 mm in diameter, bearing ovate leaves in a compact rosette.