ITV said the actress, whose roles include appearances in Dexter, The Night Porter and The Duchess, would "take a lead role" in the show but would not disclose any details about her character.
The show's creator Chris Chibnall said it had "been agony" keeping the news secret.
He said: "We have another big story to tell this year and that meant finding exceptional actors for new roles. There's none more exceptional than Charlotte: to be writing for her is a dream come true. As you can imagine, Charlotte's character is an integral part of our new story - but that's where we're drawing a veil, for now."
ITV has already confirmed that show mainstays David Tennant and Olivia Colman will return to the second series after the first wowed more than nine million viewers last year.
Tennant, who played Detective Inspector Alec Hardy, was subsequently hired for the US remake, unlike Colman who will be back in the UK version as Ellie Miller, the detective sergeant left devastated after her husband confessed to murdering schoolboy Danny Latimer.
Jodie Whittaker and Andrew Buchan, who play Danny's parents Beth and Mark Latimer, will also return to their roles, alongside Arthur Darvill as local vicar Paul Coates.