William Óge Martyn (fl. 1566–1592) was the 101st Mayor of Galway.
Also known as William Óge Martyn fitz Thomas, he was a son of Thomas Óge Martyn and Evelina Lynch of Galway. Bailiff of Galway in 1566 to 1567, he was kidnapped by the Earl of Thomond in January 1570 but was free in time to participate at the battle of Shrule in April of the same year. He served as High Sheriff of County Galway and Jailer of Athlone Castle during the 1570s and 1580s.
Martyn attempted to capture or kill the pirate Gráinne Ní Mháille at her stronghold of Rockfleet Castle in 1579, but was repulsed.
In 1580 he personally hanged, among others, William Burke, third son of the Earl of Clanricarde, for rebellion at the gallows outside the walls of Galway, fully aware that the condemned men had been pardoned.
The Annals of Loch Ce state:
LC1581.16 The Earl of Clann-Rickard's son, i.e. William Burk, went to Gaillimh to make peace with the Foreigners, on the engagement and guarantees of the Mayor, and of the town besides; and there was within before him a perpetrator of injury and destruction upon the Clann-Rickard, i.e. William Og Martin, and two bands of soldiers along with him. And after the Earl's son went in, William Martin and the Saxons acted treacherously towards him; and they apprehended himself; and nine of his people were hanged, and he himself was put in prison, in despite of the mayor, and of the town. And not long after that the Earl's son, and Toirdhelbhach, the son of Donnchadh O'Briain, were hanged; and on Corpus Christi the Earl's son was hanged, and O'Briain's son was hanged on the morrow.
Martyn served as Mayor of Galway from September 1586 to September 1587. He participated in a notorious session in December 1586, as recorded in the Annals of the Four Masters: