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.[1] 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 sap may have oozed and dried.

Botanist John Howell observed that Toxicodendron diversilobum's toxicity obscures its merits. "In spring the ivory flowers bloom on the sunny hill or in sheltered glade, in summer its fine green leaves contrast refreshingly with dried and tawny grassland, in autumn its colors flame more brilliantly than in any other native, but one great fault, its poisonous juice, nullifies its every other virtue and renders this beautiful shrub the most disparaged of all within our region."[2]

Western Poison-oak occurs only on the Pacific Coast, where it is common, and ranges from southern Canada to the Baja California peninsula. It is one of California's most prevalent woody shrubs but also climbs, vine-like, up the sides of trees, and can be found growing as single stems in grassland--often as part of early stage succession where woodland has been removed, and serving as a nurse plant for other species. The plant often occurs in California oak woodlands and Douglas-fir (Pseudotsuga) forests. Along the Avenue of the Giants in northern California, the vine form may be seen climbing many feet up the trunks of Coast Redwoods (Sequoia sempervirens). It can also be found in damp, shady areas near running water and out of direct sunlight. Any trail leading to a waterfall on California's coast may likely be home to western poison-oak. Western poison-oak can also be found in some inland mountain ranges such as the Cascades.

Western Poison-oak leaves and twigs have a surface oil, urushiol, which causes an allergic reaction. Around 15%[3] to 30%[4] of people have no allergic response, but most if not all will become sensitized over time with repeated or more concentrated exposure to urushiol

. Effects of Poison Oak are similar to those of Poison Ivy. It first causes severe itching, evolves into inflammation, non-colored bumps, and then blistering when scratched.