Chart-topper Knight, who took over from Heather Headley in the role in September, will now be with the show until 31 May 2014.
She told the Daily Mail that doing the show, which marks her West End debut, is "bloody hard work", but "so much fun at the same time".
"I've always gone on stage to sing as Beverley Knight, but I've had to learn how to play someone else," she added.
Directed by Thea Sharrock, The Bodyguard is adapted from the hit 1992 film starring Whitney Houston and Kevin Costner and features classic Whitney songs including "One Moment in Time", "Saving All My Love", "I Wanna Dance With Somebody" and "I Will Always Love You".
The Bodyguard won Best Musical at the 2013 WhatsOnStage Awards.