The actor, currently playing the lead in the West End production of Once, will play Charlie Cotton - the eldest son of Dot's no-good son Nick Cotton.
He said: "I'm delighted to be joining the cast of EastEnders, especially at what seems to be an extremely exciting time for the show. June Brown is not only an incredible actress but has also created one of TV's most iconic characters so to be working alongside her is a dream come true."
Show bosses are staying tight-lipped about potential storylines for Bennett, who starts filming this month and makes his debut in March, and whether he will take after his good-hearted grandmother or the villainous Nasty Nick.
Executive producer Dominic Treadwell-Collins said: "Declan is a genuinely raw, instinctive and intelligent actor. It's tremendously exciting for us to have him join our cast - and the double whammy of him and June Brown working together is going to be electric. With Dot feeling increasingly alone in the world, Charlie's surprise appearance means that she finally has some blood whom she can trust. But should she?"