Patersonia sericea, commonly known as purple flag[2] or silky purple-flag[3] is a species of plant in the iris family Iridaceae and is endemic to eastern Australia.

It is a densely-tufted perennial herb with linear, sword-shaped leaves, broadly egg-shaped, bluish-violet tepals and an oval capsule.

[2][3][4][5][6] Patersonia sericea was first described in 1807 by Robert Brown in Curtis's Botanical Magazine, from specimens "...furnished us by Messrs. Lee and Kennedy, of Hammersmith, West London who received the seeds, from which they raised it, from Port Jackson".

[7][8] The specific epithet (sericea) is derived from the Latin word sericus meaning "silken",[9] referring to the hairs at the base of the juvenile leaves.

[10] The names of two varieties of P. sericea are accepted by the Australian Plant Census: Patersonia longifolia was described in 1810 by Robert Brown in his Prodromus Florae Novae Hollandiae et Insulae Van Diemen[14][15] but reduced to a variety in 1893 by Charles Moore in the Handbook of the Flora of New South Wales.