The many branches of Euphorbia keithii spread, their upper parts erect, with irregularly spaced constrictions up the stems to form segments. The dark green stem segments are usually five-angled and oblong, but may have from three to six angles and become cone-shaped near the tips, not as pronounced as those of E. cooperi. The stem sections become up to 25 cm long and 4 cm wide.

The rudimentary leaves are seen only on young stem parts. They are ovate in shape, about 5 mm long and soon to drop off. The paired spines are thick, 5 mm to 8 mm long, occurring on the hard-edged stem ridges or vertically joined spine shields (Coates Palgrave, 2002; JSTOR).