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.
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? email@example.com
A stimulating conversation with a neighbour will change your outlook. It's always exciting when you realise there are more options than you previously thought. The chance to work with people from different backgrounds will be enlightening. Don't be surprised when you start valuing things you once overlooked. Putting up with toxic relationships will become a thing of the past. You'll start demanding authenticity in every area of life, especially the personal realm.More