The Girls Aloud star will play Henry's cousin Prissy Polly in the 3D movie, alongside confirmed actors Anjelica Huston and Richard E. Grant.
Walsh said: "I am so excited to have been asked! The Horrid Henry books are wonderful and to be playing the role of Prissy Polly in 3D is incredible!"
The 29-year-old previously appeared with her Girls Aloud bandmates in 2007 film St Trinian's.
Horrid Henry, produced by Vertigo Pictures and Novel Entertainment, will also feature Noel Fielding, Jo Brand, Mathew Horne and Prunella Scales.