Patersonia occidentalis, commonly known as purple flag,[2] or long purple-flag,[3] is a species of flowering plant in the family Iridaceae and is endemic to southern Australia.

It is a tufted, rhizome-forming perennial with narrow, sharply-pointed, strap-like leaves, egg-shaped, bluish violet sepals and a cylindrical capsule.

[5][6][7] Patersonia occidentalis was first formally described by the botanist Robert Brown in 1810 in his book Prodromus Florae Novae Hollandiae.

In Western Australia it grows in winter-wet areas, on sand dunes and around granite outcrops and is widespread and common between the Murchison River in the north and Israelite Bay in the south.

In Victoria the species is widespread in near-coastal areas on poorly-drained sites and in Tasmania it forms clumps in swampy places in the north and east of the state.