Walder Frey, better known as Black Walder, to differentiate him from the many other Walders in the Frey family, is a great-grandson of Lord Walder Frey and a character of A Song of Ice and Fire. His father is Ser Ryman Frey, eldest son of Ser Stevron Frey, eldest son of Lord Walder. He's the younger brother of Ser Edwyn Frey and the older brother of Petyr Frey. Black Walder is a wiry man with a pointed black beard.
Even among the Freys, Black Walder has a particularly vile reputation and is known for his foul temper. Black Walder has a particular feud with his elder brother Edwyn Frey, planning to succeed to the Twins himself.
Black Walder is believed to have been having affairs with many of the women at the Twins. He is suspected of having affairs with both his brothers' wives, his step-great-grandmother, Walder Frey's seventh wife Anarra Farring, his notoriously promiscuous cousin "Gatehouse Ami" Amerei Frey, and it is common knowledge he slept with another cousin, "Fair" Walda Frey. These cast doubts on the paternity of some of the Freys, such as Edwyn's daughter Walda, Petyr's daughter Perra, and Walder Frey's six children from his seventh wife. He refuses to marry, according to Merrett Frey.
Daven Lannister, who was betrothed to a Frey girl as part of the agreement between Tywin Lannister and Walder Frey, tells his cousin Jaime Lannister "From what Edwyn tells me, though, I'd best pick one who hasn't flowered yet, or I'm like to find that Black Walder has been there first."
During the Red Wedding Catelyn notices Black Walder acting as gentle as a lamb before the massacre begins. She later sees him hamstring a Vance who had been fighting Ser Harys Haigh. Patrek Mallister, heir to Seagard, is captured during the Red Wedding. His father, Lord Jason Mallister, holds out but is forced to surrender Seagard when Black Walder threatens to hang Patrek, after which the Mallisters are kept prisoner in their own castle.
When Black Walder's younger brother Petyr "Pimple" Frey is captured by the Brotherhood without Banners, Walder Frey's ninth son Merrett Frey offers to deliver the ransom of a hundred dragons to Oldstones. He thinks about how his father will soon die, at which a struggle for power will break out among the Freys. Edwyn and Black Walder intend to throw most of their relatives out of the Twins and Merrett hopes to remain in the good graces of the main Frey branch by delivering the ransom. However he finds Petyr already hanged and is then hanged himself by Lady Stoneheart.
Black Walder leads the search for the outlaws that hanged Petyr and Merrett at Oldstones. He manages to learn from peasants in Hag's Mire that the slayer is Lady Stoneheart, but he is unable to find her.
When Ryman Frey is sent back to the Twins by Jaime Lannister for his mismanagement of the siege of Riverrun, he is hanged close within a day's ride of the Twins, along with the three Knights and twelve men accompanying him, probably by the Brotherhood without Banners. On hearing of this, Edwyn tells Jaime Black Walder is responsible, as now only Edwyn stands between Black Walder and the Twins. However the presence of a BWB member, Tom Sevenstrings, in the camp makes this unlikely.
- It is theorized by some readers that Black Walder may be responsible for the death of his grandfather Ser Stevron Frey, heir to the Twins and one of the few decent Freys, due to Walder raising him to be a good heir. Stevron was wounded at Oxcross and though apparently fine, died in his tent a few days later. Stevron was over 60 at the time, so he may have simply succumbed to his wounds. However Edwyn's belief that Black Walder was willing to kill his father to move a step closer to inheriting the Twins and Black Walder's presence at Oxcross helps this theory.
- In the HBO Series Black Walder is merged with his bastard great-uncle Walder Rivers, who is known as "Black Walder Rivers" and is the one who cuts Catelyn's throat.
- In the series Black Walder and his brother Lothar Frey are murdered by Arya Stark and put in a pie which is fed to their father before Arya murders him. This fate befell three Freys in the books, who were put into pies and fed to two of Walder Frey's sons, Aenys Frey and Hosteen Frey, and the Boltons, by the Northern Lord Wyman Manderly due to one of his sons being murdered at the Red Wedding.