405 BC

Year 405 BC was a year of the pre-Julian Roman calendar.

• The Athenian fleet follows the Spartan admiral, Lysander, to the Hellespont. The Athenian fleet is destroyed in the Battle of Aegospotami.
• The Spartan king Pausanias lays siege to Athens. Lysander's fleet blockades Piraeus. This starves Athens.
• Theramenes tries to negotiate with Lysander. He is away for three months.

• Dionysius the Elder rises to power as the tyrant of Syracuse. He makes peace with the Carthaginian general. Carthage is in control of most of Sicily.
• Dionysius the Elder expands his power. He builds a wall around Syracuse. The Greek citizens of Naxos, Catana, and Leontini are removed from their cities. Many of them are made slaves.

• Aristophanes' play The Frogs is performed.
• Euripides' The Bacchae and Iphigeneia at Aulis are performed at the Dionysia and win first prize.

• The Erechtheum, which includes The Porch of Maidens (Caryatid Porch), is completed in the Ionian style on the Acropolis in Athens after 16 years of building.

• Philolaus, Greek mathematician and philosopher (approximate date) (b. c. 480 BC)