Stacey was last seen fleeing the country after she was exposed as the killer of Archie Mitchell. Show bosses have said her shock return will be one to remember.
"I may have left EastEnders over three years ago, but the show has also been in my heart, as has Stacey, and she has never really left my side.
"I am so excited about returning and seeing what the future holds for her. It's been a while since I set foot in Albert Square and I am looking forward to coming home."
The show's new executive producer Dominic Treadwell-Collins said, "Stacey is one of EastEnders' best-loved characters, played by one of television's finest actors, and I am tremendously excited that Lacey has decided to return home to Walford.
"Stacey's last time on the Square was iconic. But that was just the first act. There is so much more to come for Stacey — and her return will send ripples of drama through the Square."
Lacey, 25, originally appeared on the soap between 2004 and 2010, during which time she picked up more than 30 awards for her portrayal of her character.
Stacey was at the centre of some major storylines, including an affair with her father-in-law Max (Jake Wood) and the shock revelation in the EastEnders' 25th anniversary live episode that she was responsible for Archie's death.