Lwa, also called loa, are spirits in the African diasporic religion of Haitian Vodou and Dominican Vudú. They have also been incorporated into some revivalist forms of Louisiana Voodoo. Many of the lwa derive their identities in part from deities venerated in the traditional religions of West Africa, especially those of the Fon and Yoruba.

In Haitian Vodou, the lwa serve as intermediaries between humanity and Bondye, a transcendent creator divinity. Vodouists believe that over a thousand lwa exist, the names of at least 232 of which are recorded. Each lwa has its own personality and is associated with specific colors and objects. Many of them are equated with specific Roman Catholic saints on the basis of similar characteristics or shared symbols. The lwa are divided into different groups, known as nanchon (nations), the most notable of which are the Petwo and the Rada. According to Vodou belief, the lwa communicate with humans through dreams and divination, and in turn are given offerings, including sacrificed animals. Vodou teaches that during ceremonies, the lwa possess specific practitioners, who during the possession are considered the chwal (horse) of the lwa. Through possessing an individual, Vodouists believe, the lwa can communicate with other humans, offering advice, admonishment, or healing.

During the Atlantic slave trade of the 16th to 19th centuries, enslaved West Africans brought their traditional religions with them. In the French colony of Saint-Domingue, which became the republic of Haiti in the early 19th century, the diasporic religion of Vodou emerged amid the mixing of different West African traditional religions and the influence of the French colonists' Roman Catholicism. From at least the 19th century, Haitian migrants took their religion to Louisiana, by that point part of the United States, where they contributed to the formation of Louisiana Voodoo, a religion that largely died out in the early 20th century. In the latter part of that century, Voodoo revivalist groups emerged in Louisiana, often incorporating both the lwa spirits of Haitian Vodou and the oricha spirits of Cuban Santería into their practices.

Modern linguists trace the etymology of lwa to a family of Yoruba language words which include olúwa (god) and babalawo (diviner or priest). The term lwa is phonetically identical to both a French term for law, loi, and a Haitian Creole term for law, lwa. The early 20th-century writer Jean Price-Mars pondered if the term lwa, used in reference to Vodou spirits, emerged from their popular identification with the laws of the Roman Catholic Church. In the early 21st century, the historian Kate Ramsey agreed that the phonetic similarity between the terms for the law and the Vodou spirits may not be "mere linguistic coincidence" but could reflect the complex interactions of African and French colonial cultures in Haiti.

Several spelling for lwa have been used. Early 20th-century writers on Haitian religion, such as Price-Mars, usually spelled the term loi. During that century, writers like the American anthropologist Melville Herskovits favored the spelling loa, although in 2008 the historian Jeffrey E. Anderson wrote that the spelling loa was typically found in older works on the topic, having fallen out of favor with scholarly writers. The spelling lwa has been favored by more recent scholarly writers including Anderson, Ramsey, the anthropologist Karen McCarthy Brown, the scholar of religious studies Leslie Desmangles, and the Hispanic studies scholars Margarite Fernández Olmos and Lizabeth Paravisini-Gebert.

Vodou teaches that there are over a thousand lwa. They are regarded as the intermediaries of Bondyé, the supreme creator deity in Vodou. Desmangles argued that by learning about the various lwas, practitioners come to understand the different facets of Bondyé. Much as Vodouists often identify Bondyé with the Christian God, the lwa are sometimes equated with the angels of Christian cosmology. The lwa are also known as the mystères, anges, saints, and les invisibles.

The lwa can offer help, protection, and counsel to humans, in return for ritual service. They are thought of as having wisdom that is useful for humans, although they are not seen as moral exemplars which practitioners should imitate. Each lwa has its own personality, and is associated with specific colors, days of the week, and objects. The lwa can be either loyal or capricious in their dealings with their devotees; Vodouists believe that the lwa are easily offended, for instance if offered food that they dislike. When angered, the lwa are believed to remove their protection from their devotees, or to inflict misfortune, illness, or madness on an individual.

Although there are exceptions, most lwa names derive from the Fon and Yoruba languages. New lwa are nevertheless added to those brought from Africa; practitioners believe that some Vodou priests and priestesses became lwa after death, or that certain talismans become lwa. Vodouists often refer to the lwa residing in "Guinea", but this is not intended as a precise geographical location. Many lwa are also understood to live under the water, at the bottom of the sea or in rivers. Vodouists believe that the lwa communicate with humans through dreams and through the possession of human beings.