The mengdu (Jeju and Korean: 멩두), also called the three mengdu (삼멩두; sammengdu) and the three mengdu of the sun and moon (일월삼멩두; irwol sammengdu), are a set of three kinds of brass ritual devices—a pair of knives, a bell, and divination implements—which are the symbols of shamanic priesthood in the Korean shamanism of southern Jeju Island. Although similar ritual devices are found in mainland Korea, the religious reverence accorded to the mengdu is unique to Jeju.

The origin myth of the mengdu is found in the Chogong bon-puri, a major shamanic narrative in Jeju religion. According to this narrative, the original mengdu were possessed by the eponymous Mengdu triplets, the three deities who were the first to practice shamanic ritual on earth. The stylistic features of mengdu refer back to important events in the miraculous conception and lives of these gods. The implements play a critical role in ritual; both the knives and the divination implements are used to divine the will of the gods, and the bell is used to invite them into the ritual ground.

Every set of mengdu is believed to incarnate the spirits both of the heroes of the Chogong bon-puri, and of the historical human shamans who previously owned the particular set. These spirits are called the "mengdu ancestors", and are thought to intervene during rituals to help the current holder accurately ascertain the will of the gods. The mengdu are conventionally passed down from one generation to another, with the previous holder becoming the newest mengdu ancestor. The implements and the ancestors that embody them are the objects of regular worship and also feature prominently in the initiation rituals of Jeju shamanism.

Traditional Jeju religion is nowadays in decline, and there is currently a glut of mengdu sets within the traditional priesthood. At the same time, many ritual practitioners who are not trained and initiated in the traditional manner are now making their own mengdu.

The mengdu are closely associated with the Chogong bon-puri, a shamanic narrative whose recitation forms the tenth ritual of the Great Gut, the most sacred sequence of rituals in Jeju shamanism. The Chogong bon-puri is the origin myth of Jeju shamanic religion as a whole, to the point that shamans honor the myth as the "root of the gods" and respond that "it was done that way in the Chogong bon-puri" when asked about the origin of a certain ritual. It is therefore to be expected that objects as important as the mengdu should be explained by it. As with most works of oral literature, multiple versions of the narrative exist. The summary given below is based on the version recited by the high-ranking shaman An Sa-in (1912–1990) with a focus on the details relevant to the mengdu.

Jimjin'guk and Imjeong'guk, a rich couple, are nearing fifty but still have no children. A Buddhist priest visits from the Hwanggeum Temple and tells them to make offerings in his temple for a hundred days. They do so, and a girl is miraculously born. They name her Noga-danpung-agissi. When the girl is fifteen, both of her parents leave temporarily. They imprison her behind two doors with seventy-eight and forty-eight locks each and tell the family servant to feed her through a hole, so that she cannot leave the house while they are absent.

The Buddhist priest of the Hwanggeum Temple learns of the great beauty of Noga-danpung-agissi and visits the house to ask for alms. When the girl points out that she cannot leave the house, the priest takes out a bell and rings it three times, which breaks every lock. When she comes out wearing a veil of chastity, he strokes her head three times and leaves. Noga-danpung-agissi then becomes pregnant. When her parents return, they decide to kill her to restore the family's honor. When the family servant insists that she be killed instead, the parents relent and decide to expel both instead. Her father gives Noga-danpung-agissi a golden fan as she leaves.

The two decide to go to the Hwanggeum Temple, encountering various obstacles and crossing many strange bridges on the way. The servant explains the etymology of the bridges, connecting each name to the process of Noga-danpung-agissi's expulsion from the family. They eventually reach the temple and meet the priest, who banishes her to the land of the goddess of childbirth. Alone there, she gives birth to triplets who tear out of her two armpits and her breasts. Having bathed them in a brass tub, she names the three boys Sin-mengdu, Bon-mengdu, and Sara-salchuk Sam-mengdu.