Giamatti is known for his work in the films "Sideways," "Rock of Ages" and "Cinderella Man," as well as in the TV miniseries "John Adams."
He will portray Harold, the playboy brother of Cora, Elizabeth McGovern's character, on next season of "Downton Abbey," Radio Times reported Monday.
Season 4 will also see the return of Season 3 guest star Shirley MacLaine, who plays Cora and Harold's mother.
"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. 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 Christmas Day episode," Carnival Films' Managing Director Gareth Neame told Radio Times.