The actress, who played Emma Peel in the cult 1960s drama, will join fellow Brits Nathalie Emmanuel and Mackenzie Crook among the new faces in the third season of the TV adaptation of writer George R.R. Martin's books.
Martin himself revealed who would be joining the cast at Comic-Con in San Diego, California on Friday.
Former Bond girl Rigg, 73, will play Olenna Tyrell, The Queen of Thorns, while Crook will play wildling raider Orell. Emmanuel has been cast as child slave Missandei.
Other new additions include Kerry Ingram, Tara Fitzgerald, Paul Kaye and Thomas Brodie-Sangster.