Chisnall, who is currently understudying Nigel Lindsay in the title role, and EastEnders star McDermott will join Richard Blackwood, who will continue in the role of Donkey in the screen-to-stage show.
The casting of Princess Fiona, who is currently being played by Girls Aloud star Kimberley Walsh, is still to be confirmed.
Dean Chisnall's extensive West End theatre credits include Love Never Dies at the Adelphi, La Cage aux Folles at The Playhouse, Never Forget at the Savoy and on tour, Evita directed by Michael Grandage at the Adelphi and The Woman in White, directed by Trevor Nunn at the Palace.
An original cast member, he has played Farquaad’s henchman, Theolonius, and Papa Ogre in the stage musical alongside cover for Shrek.
Neil McDermott rose to prominence playing bad boy Ryan Malloy in EastEnders. His stage credits include La Cage aux Folles at the Menier Chocolate Factory, Follies at London Palladium and The Sound of Music at the London Palladium.