The leaves of Protea longifolia gave the species its name: longus (Latin) meaning long and folia (also Latin) meaning leaves; the specific name referring to the plant’s long leaves. Commonly called the strap-leaved sugarbush, there is a similar Afrikaans name, riemblaar-suikerbos (thong leaf sugar bush).
The dense, generally curving leaves ascend, linear to narrowly spathulate (spoon-shaped) and sometimes slightly falcate (sickle-shaped). The leaf base tapers, the tip is rounded to acutely or obtusely pointed. The leaves are leathery, hairless and smooth (glabrous), their pale midribs conspicuous (Rourke, 1980; Andrew, 2017).