Trillium petiolatum, the Idaho trillium, also known as the long-petioled trillium or round-leaved trillium, is a species of flowering plant in the family Melanthiaceae. It is native to the northwestern United States, in Idaho, Oregon, and Washington. The type specimen for this species was gathered by Meriwether Lewis in 1806 along the Clearwater River (originally called Koos-Koos-Kai-Kai by the Nez Perce people) during the return trip of the Lewis and Clark Expedition.