Sin (Akkadian: Sîn, Suen; Sumerian:Nanna) is a Sumerian god in Mesopotamian mythology. He is the god of the moon and the son of Enlil and Ninlil. His sacred city was Ur. The name Nanna is Sumerian for "illuminator."

Sin's chief sanctuary at Ur was named E-gish-shir-gal ("house of the great light"). His sanctuary at Harran was named E-khul-khul ("house of joys"). On cylinder seals, he is represented as an old man with a flowing beard and the crescent as his symbol. In the astral-theological system he is represented by the number 30 and the moon, symbolic of the moon's crescent that often appears next to him in Mesopotamian cylinder seals. This number probably refers to the average number of days (correctly around 29.53) in a lunar month, as measured between successive new moons.

The two chief seats of Sin's worship were Ur in the south, and Harran to the north. The cult of Sin spread to other centers, and temples of the moon-god are found in all the large cities of Babylonia and Assyria.

The "wisdom" personified by the moon-god is likewise an expression of the science of astrology, in which the observation of the moon's phases is so important a factor. The tendency to centralize the powers of the universe leads to the establishment of the doctrine of a triad consisting of Sin, Shamash, and Ishtar, respectively personifying the moon, the sun, and the planet Venus.

He was named Sin in Babylonia and Assyria, and was also worshipped in Harran. Sin had a beard made of lapis lazuli and rode on a winged bull. His wife was Ningal ("Great Lady"), who bore him Utu ("Sun") and Inanna (Inanna is recognized as being the Sumerian name for Ishtar). His symbols are the crescent moon, the bull (through his father, Enlil, "Bull of Heaven"), and the tripod (which may be a lamp-stand). An important Sumerian text ("Enlil and Ninlil")[1] tells of the descent of Enlil and Ninlil (pregnant with Nanna/Suen) into the underworld. There, three "substitutions" are given to allow the ascent of Nanna/Suen. The story shows some similarities to the text known as "The Descent of Inanna".