Cirsium altissimum is a North American species of plants in the thistle tribe within the sunflower family. Common name is tall thistle or roadside thistle.
Cirsium altissimum is, as the name implies, a tall herb, sometimes reaching as much as 400 cm (160 inches or 13 1/3 feet). It is a biennial or perennial, blooming only once before dying. Leaves are toothed or shallowly lobed, with fine spines along the edge. Sometimes there is only one flower head but more often more, with pink or purple (rarely white) disc florets but no ray florets. The species grows in prairies, open woodlands, and disturbed sites.