Acacia, commonly known as the wattles or acacias, is a large genus of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa and Australasia. The genus name is New Latin, borrowed from the Greek ἀκακία (akakia), a term used by Dioscorides for a preparation extracted from the leaves and fruit pods of Vachellia nilotica, the original type of the genus. In his Pinax (1623), Gaspard Bauhin mentioned the Greek ἀκακία from Dioscorides as the origin of the Latin name.

In the early 2000s, it had become evident that the genus as it stood was not monophyletic and that several divergent lineages needed to be placed in separate genera. It turned out that one lineage comprising over 900 species mainly native to Australia, New Guinea, and Indonesia was not closely related to the much smaller group of African lineage that contained A. nilotica—the type species. This meant that the Australasian lineage (by far the most prolific in number of species) would need to be renamed. Botanist Leslie Pedley named this group Racosperma, which received little acclaim in the botanical community. Australian botanists proposed a less disruptive solution setting a different type species for Acacia (A. penninervis) and allowing this largest number of species to remain in Acacia, resulting in the two Pan-Tropical lineages being renamed Vachellia and Senegalia, and the two endemic American lineages renamed Acaciella and Mariosousa. Although many botanists still disagreed that this was necessary, this solution was eventually officially adopted at the Melbourne International Botanical Congress in 2011.

Acacia remains a widely used common name across genera.

A number of species have been introduced to various parts of the world, and two million hectares of commercial plantations have been established. The heterogeneous group varies considerably in habit, from mat-like subshrubs to canopy trees in a forest.