The CW TV series stars Adelaide Kane (Neighbours) as a young Mary, Queen of Scots.
Roberts - best known for playing Joseph Byrne on Holby between 2005 and 2011 - has been cast as Simon Westbrook, a charming but sinister English diplomat determined to destroy Mary, says Entertainment Weekly.
Prior to his casting on Reign, Roberts appeared on The CW's Beauty and the Beast for a two-episode stint.
Joining him on his new show is Canadian actress Katie Boland, playing a feral, disfigured young woman whose past remains a mystery.
Reign will debut on October 10 in the US, following the season five premiere of The Vampire Diaries.
> Neighbours star Caitlin Stasey joins Adelaide Kane in Reign
Watch a clip from Reign below: