Linum lewisii (Linum perenne var. lewisii) (Lewis flax, blue flax or prairie flax) is a perennial plant in the family Linaceae, native to western North America from Alaska south to Baja California, and from the Pacific Coast east to the Mississippi River (USDA Plant Profile: Linum lewisii). It grows on ridges and dry slopes, from sea level in the north up to 11,000 ft (3,400 m) in the Sierra Nevada.

It is a slender herbaceous plant growing to 80 cm tall, with spirally arranged narrow lanceolate leaves 1–2 cm long. The flowers are pale blue or lavender to white, often veined in darker blue, with five petals 1-1.5 cm long.