Ungus (Onuist) mac Uurgust was King of Fortriu and of the Picts (fl 729-761).

Ungus was a man of middle years by the time he became King after a confusing series of wars and betrayals which may suggest that he had no good claim to the throne. His most lasting work was the foundation of St Andrews in Fife. He was one of the most powerful Pictish Kings and the family he brought to power remained there for much of the next hundred years.

Ungus waged war against Dalriada, destroying the western Kingdom as a force in Scotland by the 740s. He campaigned in Munster alongside King Cathal mac Finguine (d. 742), with whom Ungus claimed kinship. Against Strathclyde he was less successful, being defeated by Teudebur of Strathclyde at Mugdock. In 756 a second campaign against Dumbarton, this time with the aid of King Eadberht of Northumbria , also failed.

He was succeeded by his brother.