The year 691 BC was a year of the pre-Julian Roman calendar. In the Roman Empire, it was known as year 63 Ab urbe condita . The denomination 691 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

• King Sennacherib of Assyria razes Babylon (or 689 BC).
• Sennacherib defeats Humban-nimena of Elam in the Battle of Halule.