Gwallog ap Llaennog (several Middle Welsh variant orthographies include Gwallawc fab Lleynawc ; standard Welsh : Gwallog ap Llëenog or Llëynog) was a hero of the Hen Ogledd and probable 6th century king of the sub-Roman state of Elmet in the Leeds area of modern Yorkshire.
He joined a group of Brythonic kings, including Urien Rheged, Rhydderch Hael and Morgant Bwlch of Bryneich, in an attempt to defeat the Angles of Bernicia. This endeavour failed after Urien was slain by Llofan Llaf Difo.
Another tradition asserts that Gwallog later made war against Urien's former kingdom of Rheged in concert with Dunod Fawr of the Northern Pennines, attacking Urien's sons.
In some sources, Gwallog is given the epithet Marchog Trin, meaning "battle horseman". If his identification as king of Elmet is correct, he was succeeded by his son Ceredig, the last king of Elmet, who was deposed by St. Edwin of Deira.