The Emmy- and Golden Globe-winning actor, perhaps best known for his role as a schlumpy wine snob in Alexander Payne's Oscar-winning film, will star as Harold, Cora's "maverick, playboy brother" in the season finale.
It's the latest surprising casting news for the costume drama, which will look quite different when it returns to PBS on Jan. 14.
Following the departure of series regulars Dan Stevens, Jessica Brown Findlay and Siobhan Finneran, "Downton Abbey" will welcome a crop of new cast members that includes Dame Kiri Te Kanawa, Tom Cullen, Julian Ovenden, Nigel Harman, Joanna David and Gary Carr.
Giamatti will play the son of Shirley MacLaine, also slated to return in Season 4 following her brief guest role in Season 3.
"We're excited that Paul Giamatti will be joining us on 'Downton' to play Cora's brother Harold, the rather free-spirited uncle to Mary and Edith," said Gareth Neame, Carnival Films' managing director.
"We can't wait to see him work alongside Shirley MacLaine, who are both sure to upset the Granthams' apple cart in this year's finale."
Giamatti is no stranger to period drama, having won an Emmy for playing the title role in the acclaimed HBO miniseries "John Adams."
He also starred as comic book artist Harvey Pekar in "American Splendor" and, more recently, in David Cronenberg's "Cosmopolis."
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