Urushiol is an oily mixture of organic compounds with allergenic properties found in plants of the family Anacardiaceae, especially Toxicodendron spp. (e.g., poison oak, Chinese lacquer tree, poison ivy, poison sumac), Comocladia spp. (maidenplums), Metopium spp. (poisonwood), and also in parts of the mango tree as well as the fruit of the cashew tree. In most individuals, urushiol causes an allergic skin rash on contact, known as urushiol-induced contact dermatitis.

The name urushiol is derived from the Japanese word for the lacquer tree, Toxicodendron vernicifluum (漆, urushi). The oxidation and polymerization of urushiol in the tree's sap in the presence of moisture allows it to form a hard lacquer, which is used to produce traditional Chinese, Korean and Japanese lacquerware.