Mahonia is a genus of approximately 70 species of evergreen shrubs and, rarely, small trees in the family Berberidaceae, native to eastern Asia, the Himalaya, North and Central America. They are closely related to the genus Berberis and botanists disagree on whether to recognize a separate Mahonia. Many botanists prefer to classify Mahonia as a part of Berberis because several species in both genera are able to hybridize, and because there are no consistent morphological differences between the two groups other than the leaf pinnation (Berberis sensu stricto appear to have simple leaves, but these are in reality compound with a single leaflet and are termed "unifoliolate"; additionally their branched spines are modified compound leaves). However, recent DNA-based phylogenetic studies retain the two separate genera, by clarifying that unifoliolate-leaved Berberis s.s. is derived from within a paraphyletic group of shrubs bearing imparipinnate evergreen leaves, which are then divided into three genera: Mahonia, Alloberberis (formerly Mahonia section Horridae), and Moranothamnus (formerly Berberis claireae); a broadly-circumscribed Berberis (that is, including Mahonia, Alloberberis, and Moranothamnus) would also be monophyletic.

Mahonia species bear pinnate leaves 10–50 cm (3.9–19.7 in) long with 3 to 15 leaflets, and flowers in racemes which are 5–20 cm (2.0–7.9 in) long. Several species are popular garden shrubs, grown for their ornamental, often spiny, evergreen foliage, yellow (or rarely red) flowers in autumn, winter and early spring, and blue-black berries. The flowers are borne in terminal clusters or spreading racemes, and may be among the earliest flowers to appear in the growing season. The ripened fruits are acidic with a very sharp flavor. The plants contain berberine, a compound found in many Berberis and Mahonia species which causes vomiting, lowered blood pressure, reduced heart rate, lethargy, and other ill effects when consumed.

The genus name, Mahonia, derives from Bernard McMahon, one of the stewards of the plant collections from the Lewis and Clark Expedition. The type species of the genus is M. aquifolium.