The actress, who has been in the soap since 2004, wants to pursue a movie career.
'I've had the most fantastic time at EastEnders,' says Lacey, 22.
'Stacey's been a brilliant role - as has being part of the feisty Slater family.
'I'm so lucky to have had such challenging storylines, from the affair with Max and being diagnosed as bipolar, to the shock of being revealed as Archie's killer in the live episode.
'I'll miss everyone in Albert Square, but the time has come to try something different.'
Lacey's final scenes will air later this year.
'We'll be very sad to see Lacey leave, she's one of TV's best young actresses,' show boss Bryan Kirkwood tells The Sun. 'The door will be left open for her to return.'
Lacey's screen mum Jean, played by actress Gillian Wright, 49, is also leaving.