The Thirty Tyrants were a pro-Spartan oligarchy installed in Athens after Athens' defeat in the Peloponnesian War in April 404 BC. Its two leading members were Tharamenes and Critias, a former acolyte of Socrates.

The Thirty severely reduced the number of rights of Athenian citizens. Instead of all Athenians participating in legal functions, only a specially selected 500 could. Only 3,000 people had the right to carry weapons or receive a jury trial. Hundreds of Athenians were killed (they were ordered to drink hemlock) and thousands more were exiled.

The Thirty Tyrants were overthrown by Thrasybulus, with help from Thebes, the following year.

See also Thirty Tyrants (Roman).