A Simbi (also spelled Cymbee and Sim'bi, pl. Bisimbi or Basimbi) is a Central African guardian spirit of the water and nature in traditional Bakongo religion, as well as in African diaspora spiritual traditions, such as Hoodoo in the southern United States and Palo in Cuba. Simbi have been historically identified as water people, or mermaids, pottery, snakes, gourds, and fire. Due to the Atlantic slave trade, a large percentage of Bantu peoples were either bought or stolen from Africa to the Americas. This forced migration of over 12.5 million people is the main reason that theveneration of simbi exists today in countries, such as the United States, Brazil, Cuba, and Haiti.

While there is little written historical record of the word simbi, there is consensus that it originated within Bantu-speaking and Kongo-speaking communities and almost certainly began as a means for them to understand the spiritual nature of the world around them.

Some believe the word simbi derives from simba, a Kikongo word that means "to hold, keep, preserve. The similar phrase, isimba ia nsi, which translates to "a distinguished person in the community," was recorded in an early Kikongo dictionary in the seventeenth century. This phrase and others, such as kisímbi kinsí, which translates to "the very old person who does not die," are a few of the earliest evidences of the spiritual connection of bisimbi to the land of the living and the land of the dead.

The word basimbi also translates to "guardians" with the phrase isimba ia nsi later becoming "guardians of the land."

The Bakongo people traditionally believe that bisimbi are magically water spirits (in kikongo: nkisi mia mamba) that can appear as a person, a snake, pottery, a calabash vine, or Kalûnga, a spark of fire, similar to the spark that begot the universe in Kongo creation mythology. There have also been claims of bisimbi appearing as birds, twisted trees and mermaid-like beings. They are seen as the guardians of nature and the intermediaries who travel the Kalûnga Line between Ku Seke, the physical world of the living, and Ku Mpémba, the spiritual world of the ancestors. Bisimbi are also believed to be spiritual guides, using storytelling and oral tradition to connect the living to the ancestors and their history. The likening of living elders to the bisimbi in the phrase kisímbi kinsí highlights the importance of Bakongo elders to the spiritual well-being of the community and the passing of their beliefs from one generation to the next.

The belief that bisimbi "inhabit rocks, gullies, streams, and pools, and are able to influence the fertility and well-being of those living in the area" was translocated to the United States by enslaved Bakongo and Mbundu peoples. Because forty percent of Africans taken during the Atlantic slave trade came from Central Africa's Congo Basin, and forty percent of all enslaved people brought to South Carolina between 1733 and 1807 were people of Kongo or Ambundu descent from Angola, bisimbi became revered in the United States in Black American communities in Hoodoo tradition across the American South.

The earliest known record of simbi spirits was recorded in the nineteenth century by Edmund Ruffin who was a wealthy slaveholder from Virginia, and traveled to South Carolina "to keep the slave economic system viable through agricultural reform."

"At Pooshee plantation on the Santee Canal not too far from Woodboo, Ruffin stated that a young slave boy went to a fountain for water late at night and was very frightened by a cymbee (Simbi water spirit) who was running around and around the fountain. Although few witnesses to the appearance of cymbees were found by Ruffin, he stated that they are generally believed by the slaves to be frequent and numerous. Part of the superstition was that it was bad luck for anyone who saw one to 'tell of the occurrence, or refer to it; and that his death would be the certain penalty, if he told of the meeting for some weeks afterwards." Another occurrence from an enslaved man said simbi spirits have long hair.