Ser Gregor Clegane aka "The Mountain That Rides" is a fictional character from George R. R. Martin's A Song of Ice and Fire.

Gregor Clegane, Sandor Clegane's older brother, is a retainer to House Lannister. An enormous man, he is well known for his size, prowess in battle and cruelty.

Gregor Clegane was one of the first Lannister soldiers to enter King's Landing during its sack at the end of Robert Baratheon's rebellion. He raped and murdered Princess Elia, and murdered her children Rhaenys and Aegon, a grudge House Martell still bears at the onset of A Game of Thrones.

As a young child, Gregor thrust his brother Sandor's face into a brazier, resulting in the latter's massive scarring, because Sandor had played with one of his toys. It is also suspected that Gregor has murdered his first two wives.

Ser Gregor leads the raiding parties that Eddard Stark sends Beric Dondarrion to bring to justice. In his band is searching for Dondarrion's Brotherhood Without Banners when they kidnap Arya Stark, but not recognizing her they bring her to Harrenhal where she is put to work as a servant.

In A Storm of Swords, when Tyrion Lannister demands trial-by-combat to prove his innocence in the murder of Joffrey Baratheon, Gregor champions Cersei (Joffrey's mother) and is wounded and seriously poisoned by Oberyn Martell (the aforementioned Princess Elia's brother) despite winning the contest. At the end of the book he is left gravely ill, with Tywin Lannister ordering the Maesters to restore him to health so that he can be summarily executed as an appeasement to the Martells.

Excerpts from the forthcoming A Feast for Crows suggest that Cersei will have the defrocked Maester Qyburn attempt to revive Clegane, possibly through necromancy or some other dark art of which the Citadel would not approve.