The 77-year-old actress, sister of Warren Beatty, will play Martha Levinson, the mother of Lady Grantham (Elizabeth McGovern) in the third series of the ITV1 show.
MacLaine, who won the best actress Oscar in 1983 for Terms Of Endearment, will join the cast when they start filming next month.
She has starred in more than 50 films, had six Oscar nominations and written several best-sellers in a career that spans more than half a century.
Gareth Neame, from Carnival Films which produces the show, said: "My late grandfather directed Shirley MacLaine in Gambit in 1966 so it is a delight for me that she will be joining us on Downton Abbey.
"Julian (Fellowes) has written another brilliant character in Martha Levinson, who will be a wonderful combatant for Maggie Smith's Dowager Countess and we are excited at the prospect of Shirley MacLaine playing her."
Laura Mackie, director of drama at ITV, said MacLaine's arrival on the show was a tribute to its "success on both sides of the Atlantic".
Downton Abbey scooped the award for best television mini series at the Golden Globes earlier this month.
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