The Harry Potter and Howards End star has been linked to the part for months but has only now been officially named as Vanessa Kirby's successor.
"I'm not sure which I'm more terrified about - doing justice to the real Princess Margaret or following in [Vanessa's] shoes," she said.
"The only thing I can guarantee is that I'll be shorter."
Bonham Carter's previous royal roles include The Queen Mother in The King's Speech and The Red Queen in Alice in Wonderland.
The two-time Oscar nominee will next be seen as part of the all-star female ensemble of Ocean's 8. (BBC)