Rhamnus purshiana (Cascara Buckthorn, Cascara, Bearberry, and in the Chinook Jargon, Chittam or Chitticum; syn. Frangula purshiana, Rhamnus purshianus)</strong> is a species of buckthorn native to western North America from southern British Columbia south to central California, and inland to western Montana.

It is the largest species of buckthorn, occasionally growing up to 15 m tall, though more commonly a large shrub or small tree 5–10 m tall, with a trunk 20–50 cm in diameter. The bark is brownish to silver-grey with light splotching. The leaves are deciduous, alternate, clustered near the ends of twigs; they are oval, 5–15 cm long and 2–5 cm broad with a 0.6–2 cm petiole, dark shiny green on top, fuzzy and paler green below. The flowers are tiny, 4–5 mm diameter, with five greenish yellow petals; the flowering season is brief, disappearing by early summer. The fruit is a berry 6–10 mm diameter, bright red at first, quickly maturing deep purple or black, and containing three seeds.

It grows in moist, acidic soils in the shady side of clearings or in the marginal forest understory, near the edges of mixed deciduous-coniferous forests. It typically grows as a second-generation tree after alders have colonized a barren plot of land.

Cascara sagrada powder is an herbal remedy purported to be effective in treating a variety of medical conditions. The powder derives from the dried bark of the Cascara plant, a shrub that grows along the Pacific Coast in the United States and Canada, according to the University of Michigan Health System. Although it has been used medicinally for centuries, little evidence supports its efficacy for many uses.