The Death in Paradise and Bluestone 42 actor will play a "charming and charismatic" jazz singer named Jack Ross.
Carnival Films' managing director Gareth Neame said: "We are delighted to introduce another fantastic, dynamic character to Downton Abbey.
"His addition will bring interesting twists to the drama which we can't wait for viewers to see in Series 4".
Other new cast members include Dame Kiri Te Kanawa, Tom Cullen, Julian Ovenden, Nigel Harman, Joanna David and Dame Harriet Walter.
Downton Abbey will return for an eight-part fourth series and feature-length special at Christmas 2013.
Regulars Hugh Bonneville, Maggie Smith, Michelle Dockery and Jim Carter return, as does special guest Shirley MacLaine as Martha Levinson.