MacLaine has been unveiled as Lady Grantham's mother, Martha Levinson, ahead of filming on the hit period drama's third run, which starts in February.
The six-time Academy Award nominee is being billed as a "wonderful combatant" for Dame Maggie Smith's Dowager Countess in the Julian Fellowes-penned series.
Laura Mackie, director of drama at ITV said: "It is so exciting to have an actress of Shirley MacLaine's stature joining our brilliant Downton Abbey cast for series three and a tribute to the show's success on both sides of the Atlantic."
MacLaine's distinguished career includes roles in Terms of Endearment, Bernie, In Her Shoes and The Apartment. She also released the best-selling books Sage-ing While Age-ing and I'm Over All That - And Other Confessions.
Series three of Downton Abbey is expected to air in the autumn in the UK. The second run is currently showing on Masterpiece on PBS in the US.
Producer Carnival Films' managing director, Gareth Neame commented on the casting: "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 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."
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Watch the cast of Downton Abbey being interviewed at the National Television Awards below: