Juncus is a genus of monocotyledonous flowering plants, commonly known as rushes. It is the largest genus in the family Juncaceae, containing around 300 species.
Rushes of the genus Juncus are herbaceous plants that superficially resemble grasses or sedges. They have historically received little attention from botanists; in his 1819 monograph, James Ebenezer Bicheno described the genus as "obscure and uninviting".

The form of the flower differentiates rushes from grasses or sedges. The flowers of Juncus comprise five whorls of floral parts: three sepals, three petals (or, taken together, six tepals), two to six stamens (in two whorls) and a stigma with three lobes. The stems are round in cross-section, unlike those of sedges, which are typically somewhat triangular in cross-section.

In Juncus section Juncotypus (formerly called Juncus subg. Genuini), which contains some of the most widespread and familiar species, the leaves are reduced to sheaths around the base of the stem and the bract subtending the inflorescence closely resembles a continuation of the stem, giving the appearance that the inflorescence is lateral.