Mi·tan·ni (mĭ-tān'ē, -tä'nē) An ancient kingdom of northwest Mesopotamia extending from the bend in the Euphrates River nearly to the Tigris River. Founded probably by Aryans, the kingdom was established c. 1475 B.C. and lasted until c. 1275, when it fell to the Hittites. Mi·tan'ni·an adj. & n.