Ukraine will celebrate its independence from the Soviet Union on August 24.
The Belgian capital's bronze fountain sculpture, created in 1619, is regularly dressed in different outfits from countries around the world, in ceremonies accompanied by brass band music.
Ukrainian agency UNIAN said the boy would be given a Ukrainian traditional outfit consisting of a white woolen shirt, silk blue trousers, a red belt, a fur cap, red boots and a purse made of leather.
The costume is based on a four-centuries-old Cossack outfit, but without the pipe and tobacco pouch.
Manneken Pis received his first costume in 1698 from a governor of Bavaria. Subsequent costumes have included Elvis Presley, Father Christmas, and Mickey Mouse.