Elegia spathacea is an erect, tussock-forming perennial, from 0.3 to 0.8 m tall, with a crown diameter of 0.1 to 0.3 m. This species has compact, non-spreading rhizomes or stolon roots. Stems (culms) are round, cylindrical, unbranched, lean and rigid. Sheaths are 20–45 mm long, tan with dense reddish brown speckles, dropping off and leaving an abscission line on the stem.

Male and female flowers are produced on different plants (dioecious). The female inflorescence is 30–80 mm long, 10–20 mm wide, with 11–20 spikelets. The spathes are tan-coloured, 20–30 mm long, cartilaginous, persistent and hide the spikelets, often forming a squared, pagoda-like inflorescence. The female has fewer spikelets compared to the male inflorescence, which has 50–100 spikelets on an inflorescence that is 25–100 mm long and 10–20 mm wide. The spathes on the male inflorescence are also tan-coloured, cartilaginous and taller than the spikelets, but they do not hide the spikelets and are caducous (drop off). Elegia spathacea flowers in mid to late summer or in spring to early summer (January, February, March or October, November, December) and releases the seeds before the next flowering. The fruit is brown, club-shaped, 3-sided, about 2 × 1 mm, with a soft ovary wall; nut apex smooth.

Elegia spathacea is closely related and similar to E. rigida but E. spathacea can be distinguished by its pagoda-like inflorescence and female bracts that are longer than 20 mm.