The Battle of Opis, fought in September 539 BCE, Is a major engagement between the armies of Persia under Cyrus the Great and the Neo-Babylonian Empire under Nabonidus during the Persian invasion of Mesopotamia.

At this time, Babylonia is the last major power in western Asia that is not yet under Persian control.

The battle is fought in or near the strategic riverside city of Opis, north of the capital Babylon.

It results in a decisive defeat for the Babylonians.

A few days later, the city of Sippar surrenders to the Persians and Cyrus's forces enter Babylon apparently without a fight.

Cyrus is subsequently proclaimed king of Babylonia and its subject territories, thus ending the independence of Babylon and incorporating the Babylonian Empire into the greater Persian Empire.