The channel confirmed reports that Outnumbered star Daniel Roche has landed the lead role in the series, which is based on Richmal Crompton's novels.
The Thick Of It's Rebecca Front and Linda Green star Daniel Ryan will appear as William's parents, while Warren Clarke and Caroline Quentin will feature as new neighbours the Botts.
Denis Lawson, John Sessions and Bruce Mackinnon have signed up to play William's teachers, while Martin Jarvis, who worked on the classic Just William recordings, will narrate the series.
Isabella Blake-Thomas will play William's nemesis Violet. Other cast members include Judy Parfitt, Roy Hudd, Harry Melling and Bertie Carvel.
CBBC controller Damien Kavanagh said: "We are thrilled to have attracted such a wonderful blend of established stars and exciting new talent to breathe new life into these much-loved stories. I can't think of anyone better than Daniel Roche to bring loveable rogue William to the attention of a new generation."
Steven Andrew, CBBC's head of productions, added: "If you think Ben in Outnumbered is bad, you wait until you get a glimpse of William! We are incredibly excited to be making this production and have brought together a wonderful cast to do justice to Richmal Crompton's timeless characters and stories."
Filming for the four-part series will take place over the next five weeks and the show will air on BBC One and CBBC later this year.