This is an article about the true milk thistles. For another plant that is sometimes referred to as a milk thistle, see sow thistle.

Milk thistles are thistles of the genus Silybum Adans., flowering plants of the daisy family (Asteraceae). They are native to the Mediterranean regions of Europe, North Africa and the Middle East.

Members of this genus grow as annual or biennial plants. The erect stem is tall, branched and furrowed but not spiny. The large, alternate leaves are waxy-lobed, toothed and thorny, as in other genera of thistle. The lower leaves are cauline (= attached to the stem without petiole). The upper leaves have a clasping base. They have large, disc-shaped pink-to-purple, rarely white, solitary flower heads at the end of the stem. The flowers consist of tubular florets. The phyllaries under the flowers occur in many rows, with the outer row with spine-tipped lobes and apical spines. The fruit is a black achene with a white pappus.