Gathegi will play the recurring role of Kyle on the CW series, Entertainment Weekly reports.
A special agent for a top-secret organisation, Kyle does not suffer fools gladly and enjoys exploiting the weaknesses of his enemies.
Gathegi, 33, is best known for his film credits, including 2007's Gone Baby Gone, 2008's Twilight and its 2009 sequel New Moon. He has also appeared in episodes of House, Nikita and Veronica Mars.
The CW's president Mark Pedowitz recently predicted that Beauty and the Beast could have "a very long run" ahead, praising the "epic love story" between the show's lead characters Catherine (Kristin Kreuk) and Vincent (Jay Ryan).
"Beauty and the Beast is about love," Kreuk agreed. "Catherine sees a man [in Vincent] who has gone through so much and even though she knows he's not right for her, deep down she knows she can't walk away from him."
Beauty and the Beast received a full, 22-episode season order in the US back in November.
> Ringer's Madchen Amick to guest star on CW's Beauty and the Beast
> Beauty and the Beast casts Brothers & Sisters star Luke Macfarlane
Watch a promo for Beauty and the Beast below: