Illyrian thistle is an erect annual or biennial herb growing to a height of 2 m. Stems are erect, woolly and whitish in colour. They are very prominently winged, spiny and branched near the top. Illyrian thistle leaves are densely woolly and a whitish colour. They have deep lobes, coarse teeth, and spines on the margins. Leaf bases continue down the stems as wings. Rosette leaves can grow to 30 cm long. Illyrian thistle has purple florets with globular heads that grow to 8 cm in diameter, including numerous bracts. Bracts are large, broad, purple and end in sharp yellow spines with outer bracts reflexed.