Hypericum /ˌhaɪˈpiːrɪkəm/ is a genus of flowering plants in the family Hypericaceae (formerly considered a subfamily of Clusiaceae). The genus has a nearly worldwide distribution, missing only from tropical lowlands, deserts and polar regions. Many Hypericum species are regarded as invasive species and noxious weeds. All members of the genus may be referred to as St. John's wort, and some are known as goatweed. The white or pink flowered marsh St. John's worts of North America and eastern Asia are generally accepted as belonging to the separate genus Triadenum Raf.

Hypericum is unusual for a genus of its size because a worldwide taxonomic monograph was produced for it by Norman Robson (working at the Natural History Museum, London). Robson recognizes 36 sections within Hypericum.

Hypericum species are quite variable in habit, occurring as trees, shrubs, annuals, and perennials. Trees in the sense of single stemmed woody plants are rare, as most woody species have multiple stems arising from a single base. Shrubs have erect or spreading stems but never root from nodes that touch the ground. However, perennial herbs tend to root from these horizontal nodes, especially those that occur in wet habitats. Annual herbs tend to have taproots with a developed system of secondary hair roots. Many species of Hypericum are completely glabrous, others have simple uniseriate hairs, and some species have long, fine hairs.

Two types of glands form the characteristic punctiform (dots) patterns of Hypericum, "dark glands" and "pale glands". Dark glands consist of clusters of cells with a distinct black to reddish color. Their hue is indicative of a presence of naphthodianthrone, either hypericin or pseudohypericin, or both. These glands occur in about two-thirds of Hypericum sections and are usually restricted to certain organs. When these glands are crushed, the naphthodianthrones give a red stain. Paracelsus called the red secretions "Johannes-blut" in the 16th century, linking the plant to the martyr St. John and giving rise to the English and German common names of "St. John's wort". The pale glands, forming the pellucid dots, are each a schizogenous intracellular space lined with flattened cells that secrete oils and phloroglucinol derivates, including hyperforin. The distribution of these hypericin glands dissuades generalist herbivores from feeding on the plants. When generalist insects feed on Hypericum perforatum, 30-100% more naphthodianthrones are produced, repelling the insects.

The four thin ridges of tissue along the stems are closely to the opposite-decussate leaves of Hypericum. The ridges can be minor, just being called "ridges", or prominent, being called "wings". Terete, two-lined, and six-lined stems can occur occasionally. When a species has a tree or shrub habit, the internodes become mostly terete with age, though some trace of lines can still be detected in mature plants. The number of lines is an important distinguishing characteristic; for example, H. perforatum and Hypericum maculatum are easily confused save for H. perforatum having two lines and H. maculatum having four. The pale and dark glands are present on stems of various species, and other various species have stems without any glands. In section Hypericum, the glands are only present on stem lines, and in other sections, including Origanifolia and Hirtella, the glands are distributed across the stems.

Nearly all leaves of Hypericum species are arranged opposite and decussate, an exception being section Coridium in which whorls of three to four leaves occur. The leaves lack stipules and can be sessile or shortly petiolar, though long petioles exist in sections Adenosepalum and Hypericum. Basal articulation can be present, in which case leaves are deciduous above the articulation, or absent, in which case the leaves are persistent. Some species in sections Campylosporus and Brathys have an auricle-like, reflexed leaf base, whereas true auricles only exist in sections Drosocarpium, Thasia, and Crossophyllum. Laminar venation is highly variable, being dichotomous to pinnate to densely reticulate. Leaves are typically ovoid to elongate to linear in shape. Leaves are typically shorter than the internodes. Pale or dark glands can be present on or near the leaf margin and on the main leaf surface.

Typically, there are four or five sepals, though in section Myriandra there are rarely three. When five sepals are present they are quincuncial, and when four sepals are present they are opposite and decussate. Sepals can be equal or unequal. Sepals can be united at their base, as seen in sections Hirtella, Taeniocarpium, and Arthrophyllum. The margins are variable, having marginal glands, teeth, or hairs. The presence or absence of dark glands on the sepals is a useful distinguishing characteristic.

Almost all Hypericum petals are yellow, though a range of color exists from a pale lemony hue to a deep orangish-yellow. Exceptions include the white or pinkish petals of Hypericum albiflorum var. albiflorum and H. geminiflorum. Many species have petals that are lined or tinged with red, including the deep crimson petals of H. capitatum var. capitatum. Petal lengths can be equal or unequal. The petals are mostly asymmetrical except those of sections Adenotrias and Elodes. In those two sections, sterile bodies have developed between the stamen fascicles, working as lodicules to spread the petals of the pseudotubular flower, a specialized pollination mechanism. Nearly all species have glands on their petals; only section Adenotrias has completely eglandular petals. It has been hypothesized that the intensity of red on the petals is correlated with the hypericin content of the glands, but other pigments including skyrin derivatives can create a red color.