Coordinates: 31°56′N 45°17′E / 31.933°N 45.283°E / 31.933; 45.283

Isin (Sumerian: I₃-si-in^ki) was an ancient city-state of lower Mesopotamia about 20 miles south of Nippur at the site of modern Ishan al-Bahriyat in Iraq's Al-Qādisiyyah Governorate.

The city lay on the Isinnitum Canal, part of a set of waterways that connected the cities of Mesopotamia. The patron deity of Isin was Nintinuga (Gula) goddess of healing, and a temple to her was built there. The Isin king Enlil-bani reported building a temple to Gula named E-ni-dub-bi, a temple for Sud named E-dim-gal-an-na, a temple E-ur-gi-ra to Ninisina, as well as a temple for the god Ninbgal.

The site of Isin was occupied at least as early as the Early Dynastic Period in the middle of the 3rd millennium BC, and possibly as far back as the Ubaid period. While cuneiform tablets from that time were found, the first epigraphic reference to Isin was not until the Ur III period.

When the deteriorating Third Dynasty of Ur (Ur-III) finally collapsed at the hands of the Elamites at the end of the third millennium BC, a power vacuum was left that other city-states scrambled to fill. The last king of the Ur Dynasty, Ibbi-Sin, had not the resources nor the organized government needed to expel the Elamite invaders. One of his governmental officials, Ishbi-Erra, relocated from Ur to Isin, another city in the south of Mesopotamia, and established himself as a ruler there. One of Ishbi-Erra's year names reports his defeating Ibbi-Sin in battle.