Ariaeus (fl. 401-394 BC) was a Persian general who fought alongside Cyrus the Younger at the Battle of Cunaxa and later was involved in the assassination of Tissaphernes.
Ariaeus appears in historic records in 401 BC, in Xenophon's description of the events leading up to the Battle of Cunaxa. Xenophon noted that he was a friend of Cyrus and was said to be fond of young boys, which was why he was an intimate of the young Thessalian general Menon.
At the Battle of Cunaxa he was Cyrus' second in command and commanded the left. According to Ctesias, he was alongside Cyrus, when Cyrus succeeded in wounding Artaxerxes, but this is unlikely. This would put him on the right alongside Cyrus, and Xenophon and Diodorus both agree he was on the left, which would have been some distance away.
As soon as Ariaeus discovered that Cyrus was slain in battle, he retreated with the surviving Persian troops. After the battle he offered to wait and return with the surviving Greek soldiers. Clearchus, speaking on behalf of the Greek soldiers, who considered themselves the victors in the battle, sent him a message to offer him the throne of Persia, but he declined. The Greeks then met up with Ariaeus and his Persian troops and they both agreed not to betray each other, and Ariaeus promised to lead them safely out of Persia.
Subsequently, Tissaphernes, promising Artaxerxes he would destroy the Greek troops entirely, persuaded the king to come to terms with Ariaeus, if Ariaeus would help betray the Greeks. Tissaphernes met with Ariaeus and persuaded him to join him, and Tissaphernes and Ariaeus led the Greeks to believe that they and Artaxerxes were willing to make peace with them and lead them to safety. Clearchus, Menon and three other generals (Agis of Arcadia, Socrates of Achaea and Proxenus of Boetia) along with 20 officers and some 200 troops later met with Tissaphernes on apparently cordial terms.
At a given signal, the officers and as many of the troops as could be caught were killed and all the generals were captured. They were then taken to Artaxerxes and killed. Some of the surviving soldiers found their way to the Greek encampment and, in response to their news, the remaining Greeks started preparing to attack the Persians. Ariaeus was immediately sent to assuage the Greeks. Ariaeus told the Greeks that only Clearchus has been killed, for reason of treachery, and tried to persuade them to lay down their arms, which the Greeks were reluctant to do. The surviving Greeks eventually decided to leave the camp and find their way out of Persia and return to Greece. Ariaeus joined Tissaphernes in an unsuccessful pursuit of the Greeks.