The boy flagged down a motorist on Drottningsholmsbron, a bridge connecting Drottningholm Palace to nearby Kärsön, at about 1pm on Monday.
“It’s a bit unclear what has happened, but the boy came running, thinly clothed and cold,” said police spokesman Björn Engström.
“He had been in the water in some way or another.”
The boy was said by police to have been shocked. Police are treating the case as serious assault.