Quassia is a plant genus in the family Simaroubaceae. Its size is disputed; some botanists treat it as consisting of only one species, Quassia amara from tropical South America, while others treat it in a wide circumscription as a pantropical genus containing up to 40 species of trees and shrubs.

Quassia amara, also known as amargo, bitter-ash, bitter-wood, or hombre grande is a species in the genus Quassia, with some botanists treating it as the sole species in the genus. The genus was named by Carl Linnaeus who named it after the first botanist to describe it: the Surinamese freedman Graman Quassi. Q. amara is used as insecticide, in traditional medicine and as additive in the food industry.