Cassia is a genus of flowering plants in the legume family, Fabaceae, and the subfamily Caesalpinioideae. Species are known commonly as cassias. The genus includes 37 species and has a pantropical distribution. Species of the genera Senna and Chamaecrista were previously included in Cassia. Cassia now generally includes the largest species of the legume subtribe Cassiinae, usually mid-sized to tall trees.

Cassia is also the English common name of some unrelated species in the genus Cinnamomum of the family Lauraceae.

Cassia species occur in a range of climates. Some can be utilized widely as ornamental plants. They have been used in reforestation projects, and species from desert climates can be used to prevent desertification.

Cassia species are used as food plants by the caterpillars of many lepidopteran taxa. For example, the skipper Astraptes fulgerator and the pierids Catopsilia pomona and C. pyranthe are all seen on Cassia fistula. The latter utilizes several other cassias, as well.

The plant pathogenic viruses cassia yellow blotch bromovirus and cassia yellow spot potyvirus were first described from Cassia.

Because the name Cassia is not precise, it is sometimes difficult to know what is meant by references to plants known as "cassias". Cassia gum, for example, is made from Senna obtusifolia, a species formerly included in genus Cassia.

Genera Cassia and Senna are both known in systems of traditional medicine. Cassia fistula, for example, is used in Ayurvedic medicine.^{[citation needed]}

There exists some culinary use for cassias. The fruit of some species is edible. In Central America, its pods are stewed into a molasses-like syrup, taken as a sweetener and for its nutritional and medicinal effects. Some have toxins in their seeds, however.