The 77-year-old will join the cast when they start filming the third season next month.
She will play the mother of Lady Cora Grantham, portrayed by Elizabeth McGovern, in the third season of the hit period drama, which took Golden Globe and Emmy prizes for best miniseries or TV movie.
Gareth Neame of Carnival Films, which produces the show, said Monday that MacLaine's character will be a "wonderful combatant" for Maggie Smith's Dowager Countess.
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