The unidentified boy was seated along the side of the float -- a decorated flatbed trailer pulled by a red Chevrolet pickup truck -- with his feet dangling from the side, witness James Collins told The Portsmouth (N.H.) Herald.
He fell just as the holiday parade started.
The float -- which featured a banner reading, "Daniel Webster Council, Scouting for Food, Boy Scouts Pack 166" -- had no railing, the newspaper said.
Portsmouth Police Chief Michael Magnant called the fatality "a terrible tragedy."
NASA celebrates Earth Day with #GlobalSelfie
Lytro unveils camera that can focus a photo after shooting it