Nanna, Sīn or Suen (Akkadian: 𒂗𒍪 EN.ZU, pronounced Su'en, Sen, Sîn), and in Aramaic syn, syn’, or even shr 'moon', or Nannar (Sumerian: 𒀭𒋀𒆠 ^DŠEŠ.KI, ^DNANNAR) was the god of the moon in the Mesopotamian religions of Sumer, Akkad, Assyria, Babylonia and Aram. He was also associated with cattle, perhaps due to the perceived similarity between bull horns and the crescent moon. He was always described as a major deity, though only a few sources, mostly these from the reign of Nabonidus, consider him to be the head of the Mesopotamian pantheon.

The two chief seats of his worship were Ur in the south of Mesopotamia and Harran in the north, though he was also worshiped in numerous other cities, especially in the proximity of Ur and in the Diyala area. In Ur, he was connected to royal power, and many Mesopotamian kings visited his temple in this city.

According to Mesopotamian mythology, his parents were Enlil and Ninlil, while his wife was Ningal, worshiped with him in his major cult centers. Their children included major deities Innanna (Ishtar) and Utu (Shamash) and minor gods such as Ningublaga and Numushda. Some deities, for example Nanaya and Pinikir, were sometimes regarded as his children due to syncretism between them and his daughter Ishtar. Nanna acquired a number of syncretic associations himself, and the logographic writings of his name were used to represent these of other moon gods, such as Ugaritic Yarikh or Hurrian Kusuh.