Theramenes (thərăm´ənēz), c.455–404? BC, Athenian statesman. He helped to establish (411 BC) the oligarchical Four Hundred but was later active in overthrowing them. He fought in the Peloponnesian War, notably in the battle of Cyzicus (now in Turkey) and in the capture of Byzantium. Sent (404) to negotiate peace with Sparta, he was accused of treachery because of his deliberate waste of three months in discussion while Athens was under siege. He was elected one of the Thirty Tyrants. Critias denounced him and caused him to be put to death.