Bennett, who is currently nominated for a WhatsOnStage Award for his role as 'Guy' in the Broadway import musical, will be leaving Once on 8 March. A new cast is expected to be announced shortly.
Once has also extended at the Phoenix Theatre, where it opened in April last year, until 4 July 2015.
According to the BBC, Bennett will begin filming for EastEnders this month and can be seen on screen from March.
He'll be playing the grandson of Dot Cotton, played by June Brown.
Bennett said: "I, of course, will miss treading the boards in the West End, but I am very much looking forward inhabiting my new East End role. I wish the new Once cast as much enjoyment and pleasure as I have experienced throughout my time with the show."
Once has a book by award-winning Irish playwright and screenwriter Enda Walsh. Based on the 2007 motion picture written and directed by John Carney, music and lyrics are by Academy Award-winning Glen Hansard and Markéta Irglová who, for "Falling Slowly", won the Oscar for Best Original Song.
Bennett is not the only West End star to have swapped stage acting for the popular BBC soap recently. Danny-Boy Hatchard, who received rave reviews for his performance in the recent revival of Jonathan Harvey's play Beautiful Thing, has been signed up to play Lee, the soldier son of Danny Dyer's character Mick.