(Ἀρίσταινος), of Megalopolis, sometimes called Aristaenetus by Polybius (Schweigh. ad Polyb. 17.1) and Plutarch (Plut. Phil. 13, 17). Aristaenus, however, appears to be the correct name. He was strategus of the Achaean league in B. C. 198, and induced the Achaeans to join the Romans in the war against Philip of Macedon. Polybius defends him from the charge of treachery for having done so. In the following year (B. C. 197) he was again strategus and accompanied the consul T. Quinctius Flamininus to his interview with Philip. (Plb. 32.19_21, 32; Plb. 17.1, 7, 13.) In the same year he also persuaded the Boeotians to espouse the side of the Romans. (Liv. 33.2.) In B. C. 195, when he was again strategus, he joined Flamininus with 10,000 foot and 1000 horse in order to attack Nabis. (Liv. 34.25, &c.) He was also strategus in B. C. 185, and attacked Philopoemen and Lycortas for their conduct in relation to the embassy that had been sent to Ptolemy. (Plb. 23.7, 9, 10.)

Aristaenus was the political opponent of Philopoemen, and showed more readiness to gratify the wishes of the Romans than Philopoemen did. He was eloquent and skilled in politics, but not distinguished in war. (Plb. 25.9; comp. Plut. Phil. 17; Paus. 8.51.1.)