Under which Roman emperor did construction of the Colosseum begin?

Construction of the Colosseum began under the Roman emperor Vespasian.

Vespasian initiated the project around AD 70–72, after becoming emperor and founding the Flavian dynasty. The building occupied the site of an artificial lake created for Nero’s enormous palace complex, the Domus Aurea. Filling that private lake and replacing it with a public amphitheatre helped Vespasian present his rule as a restoration of Rome after Nero and the civil wars.

The project was not finished during Vespasian’s reign. His son Titus completed the main structure and inaugurated it with lavish games in AD 80. Vespasian’s younger son, Domitian, later added or completed important features, including the underground hypogeum used to house animals, scenery, and performers.

The emperor’s name is sometimes confused with Titus because Titus oversaw the opening, or with Nero because the arena replaced part of Nero’s former estate. Constantine was born after construction began, and Augustus ruled more than a century earlier. The monument is therefore a Flavian project begun by Vespasian and completed by his successors.