Oxalis (/ɒksˈɑːlɪs/ (British English) or /ˈɒksəlɪs/ (American English)) is a large genus of flowering plants in the wood-sorrel family, Oxalidaceae, comprising over 550 species. The genus occurs throughout most of the world, except for the polar areas; species diversity is particularly rich in tropical Brazil, Mexico, and South Africa.

Many of the species are known as wood-sorrels (also as wood sorrels or woodsorrels) as they have an acidic taste reminiscent of the sorrel proper (Rumex acetosa), which is not closely related. Some species are called yellow sorrels or pink sorrels after the colour of their flowers instead. Other species are colloquially known as false shamrocks, and some called sourgrasses. For the genus as a whole, the term oxalises is also used.

The plants are annual or perennial. The leaves are divided into three to ten or more obovate and top-notched leaflets, arranged palmately with all the leaflets of roughly equal size. The majority of species have three leaflets, superficially similar to those of some clovers. Some species exhibit rapid changes in leaf angle in response to temporarily high light intensity to decrease photoinhibition.

The flowers have five petals, which are usually fused at the base, and ten stamens. The petal colour varies from white to pink, red or yellow; anthocyanins and xanthophylls may be present or absent but are generally not both present together in significant quantities, meaning that few wood-sorrels have bright orange flowers. The fruit is a small capsule containing several seeds. The roots are often tuberous and succulent, and several species also reproduce vegetatively by production of bulbils, which detach to produce new plants.

Several Oxalis species dominate the plant life in local woodland ecosystems, be it Coast Range ecoregion of the North American Pacific Northwest, or the Sydney Turpentine-Ironbark Forest in southeastern Australia where least yellow sorrel (O. exilis) is common. In the United Kingdom and neighboring Europe, common wood sorrel (O. acetosella) is the typical woodland member of this genus, forming large swaths in the typical mixed deciduous forests dominated by downy birch (Betula pubescens) and sessile oak (Quercus petraea), by sycamore maple (Acer pseudoplatanus), common bracken (Pteridium aquilinum), pedunculate oak (Q. robur) and blackberries (Rubus fruticosus agg.), or by common ash (Fraxinus excelsior), dog's mercury (Mercurialis perennis) and European rowan (Sorbus aucuparia); it is also common in woods of common juniper (Juniperus communis ssp. communis). Some species – notably Bermuda-buttercup (O. pes-caprae) and creeping woodsorrel (O. corniculata) – are pernicious, invasive weeds when escaping from cultivation outside their native ranges; the ability of most wood-sorrels to store reserve energy in their tubers makes them quite resistant to most weed control techniques.

A 2019 study suggested that species from this genus have a symbiotic relationship with nitrogen fixing Bacillus endophytes, storing them in plant tissues and seeds, which could explain its ability to spread rapidly even in poor soils.

Tuberous woodsorrels provide food for certain small herbivores, such as the Montezuma quail (Cyrtonyx montezumae). The foliage is eaten by some Lepidoptera, such as the Polyommatini pale grass blue (Pseudozizeeria maha), which feeds on creeping wood sorrel and others, and dark grass blue (Zizeeria lysimon).

Oxalis species are susceptible to the rust fungus (Puccinia oxalidis).