Season 2 of the ITV series is airing in the United States Sundays on PBS under the network's Masterpiece programming banner.
Best known for her work in "Steel Magnolias," "Terms of Endearment" and "The Apartment," MacLaine will play Martha Levinson, the mother of Elizabeth McGovern's character Lady Grantham when Season 3 begins production this spring.
"Shirley MacLaine is a great actress and she's as American as the day is long," Masterpiece Executive Producer Rebecca Eaton said in a statement. "I can't wait to see her go toe-to-toe with Maggie Smith's Lady Violet."
Carnival Films' Managing Director Gareth Neame said: "My late grandfather (Ronald Neame) directed Shirley MacLaine in 'Gambit' in 1966 so it is a delight for me that she will be joining us on 'Downton Abbey.' ['Downton' creator] Julian [Fellowes] has written another brilliant character in Martha Levinson."