The Blackfish is Catelyn Stark's uncle, whose skills as a warrior are of great use to Robb Stark in his continuing war efforts with the Lannisters.
Russell is the first significant casting for the third season, which is expected to include a number of new names when the show returns to filming later this month.
The British actor has starred previously in Boys from the Blackstuff, Silent Witness, Cracker, and as Captain Tanner in the recent Sherlock Holmes movies.
Game of Thrones stars Alfie Allen (Theon Greyjoy), Emilia Clarke (Daenerys Targaryen), Michelle Fairley (Catelyn Stark), Kit Harington (Jon Snow), Lena Headey (Cersei Lannister) and Richard Madden (Robb Stark), and the author of the original novels George R R Martin, will appear at a panel event at San Diego's Comic-Con next week to reveal more details about the third season.
The third run will feature ten episodes and will be adapted from parts of George R R Martin's A Storm of Swords book. Other new characters set to join the series include Edmure Tully, Mance Rayder, Daario Naharis, Myranda Royce, Lady Selyse Florent and Jojen Reed and Meera Reed.
Game of Thrones season three will air in 2013 on HBO in the US and on Sky Atlantic in the UK.