Euphorbia peplus (petty spurge, radium weed, cancer weed, or milkweed) is a species of Euphorbia, native to most of Europe, northern Africa, and western Asia, where it typically grows in cultivated arable land, gardens, and other disturbed land.

Outside of its native range it is very widely naturalised and often invasive, including in Australia, New Zealand, North America, and other countries in temperate and sub-tropical regions.