According to TV Line, the Welsh actor will act as a new love interest for Lady Mary, after her husband Matthew Crawley was seemingly killed during the series's 2012 Christmas special.
Cullen will play Lord Gillingham, an old friend of the Crawley family.
Dame Harriet Walters will also appear during the new run playing an old friend of Maggie Smith's Dowager Countess, while former EastEnders star Nigel Harman will also join as a visiting valet named Green.
Earlier today (March 2) Siobhan Finneran announced that she is quitting her role as scheming maid Miss O'Brien in the show.
Jessica Brown Findlay, who played Lady Sybil, and Matthew Crawley actor Dan Stevens also left the show during its third series.