PSP police said the boy was reported missing by friends on Saturday. The circumstances surrounding the incident are unknown, but the boy was found after police launched a search for him.
National media said the boy was Romanian and his body was found submersed in around two metres of water at the bottom of a lift shaft in a building still under construction on Europa Avenue in the heart of urban Viseu.
It was removed and taken for autopsy.
PJ police, local fire-fighters and PSP officers were at the scene where a health delegate confirmed the death.