Fiennes joins Will Arnett, who is reprising the Caped Crusader role he originated in "Lego Movie", Michael Cera, voicing the hero's sidekick, Robin, and Rosario Dawson, who is playing Batgirl.
Zach Galifianakis is in negotiations to play the Joker, said The Hollywood Reporter.
Alfred has been a Batman fixture since 1943.
Chris McKay, the animation supervisor on Lego Movie, is directing the project, which has a script written by author-screenwriter Seth Grahame-Smith.
Dan Lin, Roy Lee, Phil Lord and Christopher Miller, the custodians of the Lego screen brand, are producing.
"Lego Batman" will be his first movie voice work since the 2005 Wallace & Gromit feature, "The Curse of the Were-Rabbit".
