It has a single long, narrow, fleshy leaf and up to nine purplish blue to mauve flowers with a deeply notched lobe on top of the anther.

[2][3][4] Thelymitra holmesii was first formally described in 1933 by William Henry Nicholls from a specimen collected near Portland and the description was published in The Victorian Naturalist.

[5][6] The specific epithet (holmesii) honours "Murray Holmes, a youthful and energetic orchidologist".

[6] The blue star sun orchid grows in winter wet or swampy places, sometimes in disturbed areas, forest woodland or heath.

[2][3][4][7][8] Thelymitra holmesii is listed as "vulnerable" in South Australia and as "rare" under the Tasmanian Threatened Species Protection Act 1995.