Hypericum perforatum, commonly known as St. John's wort (sometimes perforate St. John's wort or common St. John's wort), is a flowering plant in the family Hypericaceae. It is a hairless, perennial herb with woody roots, yellow flowers marked by black glands, and leaves that appear perforated due to translucent glands, producing thousands of seeds per plant.

H. perforatum is the type species of its genus, known for its historical use in folklore and traditional medicine. Probably a hybrid between the closely related H. attenuatum and H. maculatum (imperforate St. John's wort) that originated in Siberia, the species has spread worldwide. It can further hybridize with related species due to its allopolyploid nature. It is native to much of Europe, West and Central Asia, and parts of Africa and China and has been widely introduced elsewhere, thriving in well-drained, temperate habitats such as meadows, hillsides, and open woods with moderate rainfall and mild temperatures. It is a resilient, toxic, and invasive plant that reproduces sexually and vegetatively, supports specialized insect herbivores, suffers from plant diseases, and poses ecological and agricultural threats in many parts of the world.

H. perforatum has been used for centuries in traditional medicine, especially for treating wounds and depression. To prepare it for use, the oil from its glands can be extracted or its above-ground parts can be dried and ground into a powder called herba hyperici. H. perforatum exhibits antidepressant effects comparable to drugs with fewer side effects for mild to moderate depression (for which it is approved in the European Union); however, it may interact with various medications by accelerating their metabolism.

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