in Mesopotamian religion, the god of the moon. Sin was the father of the sun god, Shamash (Sumerian: Utu), and, in some myths, of Ishtar (Sumerian: Inanna), goddess of Venus, and with them formed an astral triad of deities.

Nanna, the Sumerian name for the moon god, may have originally meant only the full moon, whereas Su-en, later contracted to Sin, designated the crescent moon. At any rate, Nanna was intimately connected with the cattle herds that were the livelihood of the people in the marshes of the lower Euphrates River, where the cult developed. (The city of Ur, of the same region, was the chief centre of the worship of Nanna.) The crescent, Nanna's emblem, was sometimes represented by the horns of a great bull. Nanna bestowed fertility and prosperity on the cowherds, governing the rise of the waters, the growth of reeds, the increase of the herd, and therefore the quantity of dairy products produced. His consort, Ningal, was a reed goddess. Each spring, Nanna's worshipers reenacted his mythological visit to his father, Enlil (Ea), at Nippur with a ritual journey, carrying with them the first dairy products of the year. Gradually Nanna became more human: from being depicted as a bull or boat, because of his crescent emblem, he came to be represented as a cowherd or boatman.

Sin was represented as an old man with a flowing beard—a wise and unfathomable god—wearing a headdress of four horns surmounted by a crescent moon. The last king of Babylon, Nabonidus (reigned c. 556–539 BC), attempted to elevate Sin to a supreme position within the pantheon.