The "John Adams" star will play Harold Levinson, the brother to Cora Crawley, who is played by Elizabeth McGovern. Levinson is described as a playboy and maverick.
Guest star Shirley MacLaine is set to return for the season four finale alongside Giamatti. MacLaine plays Crawley's mother Martha Levinson.
"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 Grantham's apple cart in this year's finale," executive producer Gareth Neame said in a statement.
Other new cast members for include Tom Cullen, Julian Ovenden, Nigel Harman and Gary Carr. Season four airs Jan. 5, 2014 on PBS.