Toxicodendron diversilobum (syn. Rhus diversiloba; Western Poison-oak or Pacific Poison-oak) is a plant best known for its ability to cause allergic rashes after contact. Western Poison-oak is found only on the Pacific Coast of the United States and of Canada. It is extremely common in that region, where it is the predominant species of the genus; the closely related Atlantic Poison-oak (T. pubescens) occurs on the Atlantic Coast. The hyphenated form "Poison-oak" is used, rather than "Poison Oak" to clearly indicate that it is not a variety of oak, just as "Poison-ivy" is not a variety of ivy.

Western Poison-oak is extremely variable in growth habit and leaf appearance. It grows as a dense shrub in open sunlight, or as a climbing vine in shaded areas. Like Poison ivy, it reproduces by creeping rootstocks or by seeds. The leaves are divided into three leaflets, 3.5 to 10 centimeters long, with scalloped, toothed, or lobed edges- generally resembling the leaves of a true oak, though the Western Poison-oak leaves will tend to be more glossy. Leaves are typically bronze when first unfolding, bright green in the spring, yellow-green to reddish in the summer, and bright red or pink in the fall. White flowers form in the spring and, if fertilized, develop into greenish- white or tan berries. Toxicodendron diversilobum is winter deciduous, so that after cold weather sets in the stems are leafless and bear only the occasional cluster of berries. Without leaves, poison oak stems may sometimes be identified by occasional black marks where its milky sap may have oozed and dried.

In some cases, the best treatment is to do nothing to the area.

Corticosteroid skin creams or ointments may reduce inflammation. Carefully follow the instructions when using these creams, because overuse, even of low-strength over-the-counter products, may cause a troublesome skin condition. In severe cases, systemic corticosteroids may be needed to reduce inflammation. These are usually tapered gradually over about 12 days to prevent recurrence of the rash. In addition to or instead of corticosteroid skin treatment, your doctor may prescribe tacrolimus ointment or pimecrolimus cream.

Wet dressings and soothing anti-itch (antipruritic) or drying lotions may be recommended to reduce other symptoms.

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