The Oscar-nominated actor will play Cora Crawley's brother Harold Levinson in the fourth season of the hit ITV period drama.
Paul's character Harold - who is also the uncle of Lady Mary Josephine (Michelle Dockery) and Lady Edith Crawley (Laura Carmichael) - is described as a maverick playboy.
The show's executive producer Gareth Neame said Harold is "free-spirited" and promises that both he and Martha will "upset the Granthams' apple cart" in the finale.
Paul - who will next be seen on the big screen in 'Romeo And Juliet' (written by Downton Abbey's scribe Julian Fellowes) and Steve McQueen's 'Twelve Years A Slave' - is the latest addition for the fourth season.
He joins newcomers such as Dame Kiri Te Kanawa, Dame Harriet Walters, Nigel Harman, Tom Cullen, Julian Ovenden and Gary Ross.
Hugh Bonneville, Dame Maggie Smith, Michelle Dockery and Jim Carter will all reprise their roles for the show's fourth run, which is expected to return on ITV in the autumn before airing on US screens in January 2014.