Critias was an Athenian aristocrat, poet, and thinker of the fifth century BCE. He is most infamously remembered as the leading and most violent member of the Thirty Tyrants, the brutal oligarchic regime that ruled Athens after its defeat in the Peloponnesian War. Critias was killed in 403 BCE when democratic forces overthrew the Thirty.

In his youth, he was associated with the philosopher Socrates and was a student of the sophist Gorgias. His literary works survive only in fragments, including tragedies, elegies, prose works on constitutions, and political speeches. One fragment, from a satyr play called Sisyphus, is particularly significant. In it, a character argues that the gods were a clever invention by a shrewd man to frighten people into good behavior, even in secret. This fragment is considered a key document for understanding radical sophistic theories about religion, law, and society.

Critias remains a controversial figure. His political life made him an archetype of the cruel tyrant in Athenian history. Philosophically, he is grouped with the sophists, and his surviving writings provide insight into the oligarchic admiration for Spartan discipline and the critical examination of traditional beliefs during his era.