Opis (to the Greeks; Upi in Akkadian) was a major trading city on the River Tigris in northern Babylonia, in existence since the 13th century BCE. It stood at the eastern end of the "Median Wall" (opposite Sippar on the western end) built by Nebuchadnezzar II of Babylon in the 560s BCE.
In 539 BCE Opis was the site of the battle which allowed Cyrus the Great of Persia to conquer Babylonia. In 324 BCE Alexander the Great had to deal with a major mutiny of his army there (Arrian, Anabasis 7.8-11). In the Hellenistic era Opis declined, as the new city of Seleukia, located on the opposite side of the Tigris a few miles to the south, usurped its economic importance (RE 18.1, cols.683-5).