Police say the 18-year-old fisherman was eaten alive after he jumped from his canoe into the water in Rosario del Yata, 400 miles north of Bolivian capital La Paz, last week.
Local cop Daniel Cayaya said the man was thought to have been drunk at the time.
Cayaya said the young man bled to death after the attack.
Mr Cayaya revealed the police suspect suicide because the teenager was a fisherman in the region who knew the Yata River, and its inhabitants, well.
Despite their fearsome reputation, fatal attacks by piranhas on humans are rare.