It has a single grass-like leaf and up to five white flowers Thelymitra longifolia is a tuberous, perennial herb with a single ribbed, linear to lance-shaped leaf 50–380 mm (2–10 in) long and 10–40 mm (0.4–2 in) wide.

[2][3][4] Thelymitra longifolia was first formally described in 1776 by Johann Reinhold Forster and his son Georg from a plant collected during James Cooks second voyage to the Pacific and the description was published in Characteres Generum Plantarum.

[7]: 340 The white sun orchid usually grows in sunny places in open areas amongst scrub, but sometimes in dense forest.

It is found on the North, South, Stewart, Three Kings, Chatham and Auckland Islands.

[2] It has also been reported from Norfolk Island but there is doubt that those collections match the type specimen.