Conothamnus trinervis is an erect or straggly shrub that typically grows to a height of and has thick, stiff branches. The leaves are usually arranged in opposite pairs, sometimes in whorls of three, long with three veins and a sharp point on the tip. The flowers are yellow, cream-coloured or white, occasionally purple and arranged in heads about across. Each group of three flowers has a bract at its base and the flowers have five sepals and five petals. Flowering occurs between August and October.