Athenian politician and general, active in the last years of the Peloponnesian War (431–404 BC) and controversial in his own lifetime and since. His father, Hagnon, a contemporary of Pericles, served repeatedly as one of the 10 annual generals of Athens.

In 411 Theramenes emerged as one of the revolutionary leaders who persuaded the Athenian assembly to suspend the traditional institutions and entrust supreme control temporarily to a Council of Four Hundred. The total defeat of the Athenian expedition to Sicily (415–413) and the consequent revolts of many of the subject-allies had weakened Athenian finances severely; the acknowledged purpose of the revolutionary movement was to revise the constitution on a more economical basis. But the Council of Four Hundred was only able to maintain itself for rather less than four months. It failed to win the allegiance of the main Athenian fleet, which was stationed at Samos. Divisions arose within the Council, and, in the autumn of 411, when a mutiny broke out among the troops who were fortifying Piraeus, the harbour of Athens, the Council sent Theramenes to quell it. Instead, he put himself at the head of the mutineers. The ensuing meeting of the assembly deposed the Council and restored the traditional constitution in large part but restricted some of the privileges of citizenship to a body called the Five Thousand.

In 410, commanding 20 ships, Theramenes collaborated with Alcibiades and the main Athenian fleet in inflicting a complete defeat on the Peloponnesian fleet near Cyzicus on the shore of the Propontis (Sea of Marmara). Alcibiades installed a garrison at Chrysopolis under Theramenes to exact a tithe from all shipping that came from the Black Sea. This revenue enabled the Athenians to put an end to the regime of the Five Thousand and restore their traditional institutions in full.

In 406, as captain of a ship, Theramenes took part in the naval victory over the Peloponnesians at the islands of Arginusae off the west coast of Asia Minor. Returning to Athens after the battle, he led agitation against the eight generals who had commanded in the engagement; the six who returned to Athens were condemned for negligence in not having picked up survivors from the ships disabled in the battle and were executed.

In the winter of 405–404, when the Peloponnesians besieged Athens, Theramenes had himself dispatched to negotiate with Lysander. He stayed away for three months while Athens was being reduced to starvation. Then he headed the embassy that negotiated the terms of capitulation to the Spartans.

Theramenes was a leading member of the Board of Thirty, the so-called Thirty Tyrants, whom Lysander set up to rule the conquered city soon after the capitulation. A split developed between Theramenes and Critias, another of the leaders. Critias induced the Thirty to put Theramenes to death by forcing him to drink hemlock.