It is a tall orchid with a single, purplish to blackish, tubular leaf and up to thirty or more relatively large, greyish-purple to brownish-purple flowers.

Prasophyllum triangulare is a terrestrial, perennial, deciduous, herb with an underground tuber and a single fleshy, purplish to blackish, tube-shaped leaf 250–350 mm (9.8–14 in) long and 2–5 mm (0.08–0.2 in) wide.

[2][3][4] Prasophyllum triangulare was first formally described in 1882 by Robert D. FitzGerald and the description was published in The Gardeners' Chronicle.

[3] The dark leek orchid grows in shrubland, woodland and forest between Augusta and Albany in the Avon Wheatbelt, Esperance Plains, Jarrah Forest and Warren biogeographic regions.

[2][3][4][7] Prasophyllum regium is listed as "Not Threatened" by the Western Australian Government Department of Parks and Wildlife.