Despite Kim Medcalf being the last actress to play the character, the 35-year-old is due to return to filming with the soap this summer.
The Mitchell sibling was last seen escaping to Brazil pending the trial for Den Watts's murder.
Speaking of her comeback, Westbrook said: "I'm really happy - it feels just like coming home.
"I'm really looking forward to working with Barbara, Steve, Sid and Patsy and I'm over the moon to be working with the new boss Diederick Santer as he's done such great things in turning the show around. I can't wait to get the scripts to see what Sam has been up to since she was last in the Square."
Executive producer Diederick Santer said: "With all the turmoil facing the Mitchells this year, it feels like a great time to bring back Sam, the original Mitchell Sister. What will Ronnie and Roxy make of her? Will Peggy and Phil be pleased to see her? And will Ricky's eternal devotion to Bianca be tested by the reappearance of his glamourous ex-wife?"
He added: "I'd like to take this opportunity to thank Kim Medcalf for her great work playing Sam Mitchell for three years and welcome Danniella back to pick up the baton once again."
Last month, it was rumoured that producers at the Walford soap were in talks with Westbrook in the hope of penning a return which would be "the talk of soapland for months".
Another of the soap's returnees Patsy Palmer, who plays Bianca Jackson, recently revealed that she would love for Danniella to return to the show.