The man, who's not been named, was taking part in a parade in Nijar, a town in Andalusia, Southern Spain.
Eyewitnesses say he hit his head when he fell from the throne.
The Mayor of Nijar, Antonio Jesús RodrÃguez, said the man suffered a "strong blow".
Torracardenas Hospital confirmed the man died from his injuries, after falling on Monday.
Authorities in Nijar have announced three days of mourning for the young man.
Many areas of Spain celebrate the night before Epiphany every year, to commemorate the Three King's arrival to Jesus bearing symbolic gifts.
Epiphany, on January 6th, itself is a public holiday in Spain.