Theramenes, a political leader at Athens toward the end of the 5th century B. C, born in Cos. In 411 he became a member of the council of 400; but he deserted it and beeame one of the leading agents in its overthrow. In 410 he joined the fleet under Thrasybulus, and took part in the battle of Cyzicus; and in 408 he participated in the siege of Chalcedon and the capture of Byzantium, under Alcibiades. He was one of the inferior generals at the battle of Arginusae in 406; and it was chiefly through his influence that six of the commanders were condemned to death for not saving the drowning crews, although, as they asserted, he had himself been sent with others to perform that office. During the siege of Athens by the Spartan general Lysander, when the city was reduced to great extremity, Thera-menes was sent as envoy to the Lacedaemonians. He remained three months with Lysander, who he pretended detained him that length of time without informing him that the ephors only had power to grant peace; and upon his return to the city, which was now suffering under a terrible famine, he was sent back to make peace on any terms.
The hard conditions imposed by the Lacedaemonians were assented to (see Greece, vol. viii., p. 195), and in 404 Theramenes, who during his three months' stay with Lysander had made arrangements with the Athenian oligarchical exiles, was among the most active in subverting the constitution, and became one of the thirty tyrants. He warmly supported the first measures of the government in crushing the democracy and putting to death its prominent leaders; but he afterward opposed the violent measures of Critias and his colleagues. His party daily increased; but Critias, after charging him with being a public enemy, caused him to be dragged off to prison by partisans with concealed daggers whom he had brought into the senate house, and compelled him to drink the hemlock.