Pterostylis tenuissima, commonly known as the swamp greenhood, is a species of orchid endemic to the southern mainland of Australia.

[3][4][5][6][7] Pterostylis tenuissima was first formally described in 1950 by William Nicholls from a specimen collected near Nelson and the description was published in The Victorian Naturalist.

[1][8] The specific epithet (tenuissima) is the superlative form of the Latin word tenuis meaning "thin",[9] hence "thinnest".

The swamp greenhood occurs in the far south-eastern corner of South Australia and south-western Victoria with a disjunct population on Wilsons Promontory.

It grows in swampy mud under dense thickets of woolly tea-tree (Leptospermum lanigerum) and scented paperbark (Melaleuca squarrosa), sometimes forming large colonies.