| Gartnait son of Donuel | |
| Succession: | King of the Picts |
| Reign: | 657–663 |
| Predecessor: | Talorgan I |
| Successor: | Drest VI |
| Death Date: | 663 |
| Father: | Possibly Domnall Brecc |
Gartnait son of Donuel (Irish, Old (to 900);: Gartnait mac Domnaill or Irish, Old (to 900);: Gartnait mac Dúngail|label=none; died 663) was king of the Picts from 657 until 663.
He succeeded Talorgan son of Eanfrith on the latter's death in 657. Like his predecessor Talorgan and his successor Drest son of Donuel, Gartnait reigned as a puppet king under the Northumbrian king Oswiu. Gartnait and Drest may have been sons of Domnall Brecc, who was king of Dál Riata from c. 629 until he was killed in 642.
The Northumbrian writer Bede implies that Oswiu subdued "the greater part of the Picts" in 658, suggesting Oswiu launched an offensive against the Picts after the death of his nephew Talorgan in 657. The Pictish Chronicle king lists give Gartnait a reign of five, six or six and a half years, corresponding with the notice of his death in the Annals of Ulster and the Annals of Tigernach in 663.
The king lists record that he was succeeded by his brother Drest, though Oswiu may have forced an interregnum on the kingdom from 663 to 666. Gartnait son of Donuel may be the Gartnait whose family are recorded by the Annals of Ulster and the Annals of Tigernach going to Ireland in 668.