Asclepias crassinervis is an erect herb, possibly an annual, reaching heights from 13 cm to 28 cm. Plant parts exude milky sap when damaged. The specific name, crassinervis, means thick veins.

The opposite, stalkless leaves are narrow and linear to lanceolate, tapering to acutely pointed tips. The midrib is conspicuous and pale cream, the blades hairy. Leaves become about 9 cm long.

The species distribution is in Mpumalanga and Swaziland. The habitat is probably grassland. The species is not considered to be threatened in its habitat early in the twenty first century, although the plant is rare (Leistner, (Ed.), 2000; iNaturalist; http://redlist.sanbi.org).