Piyassili (died ca. 1315 BC) was a Hittite prince, and a middle son of King Suppiluliuma I; younger than the heir Arnuwanda II, but older than the eventual successor Mursili II and probably older than the doomed Zannanza too. After Suppiluliuma I concluded a treaty with Shattiwazza, son of King Tushratta of Mitanni, and married one of his daughters to him, Piyassili led a Hittite army that put Shattiwazza on the throne of Hanigalbat. According to Hittite sources, Piyashshili and Shattiwazza crossed the Euphrates at Carchemish, then marched against Irridu, already in Hurrian territory. After having reduced Irridu and Harran, they continued east towards Washshukanni and perhaps conquered the capital Taite as well.
After Shattiwazza had been made a vassal ruler of Hanigalbat, Suppiluliuma gave to Piyassili the Akkadian name Sarri-Kusuh and the territory of Ashtata (with the cities of Ekalte, Ahuna and Terqa) and Carchemish, formerly belonging to Hanigalbat. "And all of the cities of the land of Carchemish, Murmurik, Shipri, Mazuwati and Šurun – these fortified cities– I gave to my son." (Suppililiuma-Shattiwazza treaty § 13). In fact, the whole former territory of Hanigabat west of the Euphrates seems to have come under direct Hittite rule and was governed by Piyashshili.
When the Egyptians attacked Kadesh (Kinza), Suppililiuma besieged and retook the town. The people were deported and Suppililiuma made Piyashshili governor of that town as well. Still bearing the name "Sarri-Kusuh", in ca. 1320 BC Piyashshili came to Mursili's aid in the campaign against Arzawa. Piyassili then returned to Carchemish.
Piyashshili fell ill and died before year 9 of Mursili II. After his death, a rebellion broke out in Kadesh and Nuhashshe. After it had been quelled, Piyassili's son was made king of Carchemish by his uncle Mursili II.