Year 427 BC was a year of the pre-Julian Roman calendar.
|Millennium:||1st millennium BC|
|Centuries:||6th century BC – 5th century BC – 4th century BC|
|Decades:||450s BC 440s BC 430s BC – 420s BC – 410s BC 400s BC 390s BC|
|Years:||430 BC 429 BC 428 BC – 427 BC – 426 BC 425 BC 424 BC|
- Sparta's King Archidamus II is succeeded by his son Agis II.
- Cleon says Mytilene must be destroyed. He decides not to kill the people.
- Plataea surrenders to the Spartans and Thebans. Over 200 prisoners are put to death and Plataea is destroyed.
- The civil war in Corcyra results in a victory of the democrats over the oligarchs.
- The Quaestorship is opened to the Plebs.
- for more information, see Category:427 BC births.