Syennesis (Ancient Greek: Συέννεσις) was a ruler of ancient Cilicia in the 5th century BCE.
Synnesis was a contemporary of Artaxerxes II of Persia, and when Cyrus the Younger, marching against Artaxerxes in 401 BCE, arrived at the borders of Cilicia, he found the passes guarded by Syennesis, who, however, withdrew his troops on receiving intelligence that the force advanced by Cyrus under Meno had already entered Cilicia, and that the combined fleet of the Lacedaemonians and the prince, under Samius and Tamos, was sailing round from Ionia.
When Cyrus reached Tarsus, the Cilician capital, he found that Meno's soldiers had sacked the city, and commanded Synnesis to appear before him. Syennesis had fled for refuge to a stronghold among the mountains, but he was induced by his wife, Epyaxa, to obey the summons of Cyrus. Here he received gifts of honor from the Cyrus, whom he supplied in his turn with a large sum of money and a considerable body of troops under the command of one of his sons.
At the same time, however, Syennesis took care to send his other son to Artaxerxes, to represent his meeting with Cyrus as having been something he'd been forced to do, while his heart all the time was with the king, Artaxerxes. From Xenophon's telling it appears that Syennesis at this time, though really a vassal of Persia, affected the tone of an independent sovereign.