Pterostylis williamsonii, commonly known as the brown-lip leafy greenhood, is a plant in the orchid family Orchidaceae and is endemic to Tasmania. Flowering plants have up to seven transparent green flowers with darker green and brown bands and a hairy, insect-like labellum with a blackish stripe. Non-flowering plants have a rosette of leaves on a short stalk but flowering plants lack the rosette, instead having five to seven stem leaves.
Pterostylis williamsonii, is a terrestrial, perennial, deciduous, herb with an underground tuber. Non-flowering plants have a rosette of between four and six dark green, egg-shaped leaves on a stalk long, each leaf long and wide. Flowering plants have up to nine transparent green flowers with darker green and brown bands on a flowering spike high. The flowering spike has five or six lance-shaped stem leaves which are long and wide. The flowers are long, wide. The sepal and petals are fused, forming a hood or "galea" over the column with the dorsal sepal having a brown tip. The lateral sepals turn downwards, are long, wide and have a narrow tip about long which is orange-brown on its end. The labellum is insect-like, long, about wide and creamy yellow to dark chocolate brown with a black central stripe. Flowering occurs from April to July.
Pterostylis williamsonii was first formally described in 1998 by David Jones and the description was published in Australian Orchid Research from a specimen collected by Ron and Kath Williamson at Coles Bay. The specific epithet (williamsonii) honours Ronald Herbert Williamson (1931-2003), who collected the type specimen.
The brown-lip leafy greenhood is widespread in Tasmania where it grows in forest near low shrubs and bracken.