In Sumerian mythology, Nanna (short for Nan.Nar) was the first-born son of Enlil. He was also known by the name Sin. Ninlil was his mother. He was the half-brother of Ninurta, who outranked him in the Sumerian rules of succession, as Ninurta's mother, Ninhursag, was the half-sister of Enlil. (In other words, both of Ninurta's parents were children of Anu, were only one of Nanna's parents was a child of Anu).
Nanna was the ruler of Ur, who at a given moment in time was banished from the city, probably because he didn't respect the rules of succession and tried to usurp rulership.
He is the father of Inanna and Ereshkigal.