The baby was found on a beach in the Costa Dorada resort of Roda de Bera, Tarragona.
Police were alerted when the body was spotted by a municipal cleaner at around 11.30am local time this morning.
It is thought that the boy’s body was missing other parts besides his head and that he washed up on the beach from the water.
The area was cordoned off while Civil Security officers were investigating.
According to local media, Roda de Bera Mayor Pere Virgili said the teenager may have been from a migrant boat that capsized in the Mediterranean.
A spokesperson for the Civil Guard said: “The only thing we can confirm at this time is that a baby’s body was found on Roda de Bera beach in Tarragona state. The alarm was set at 8:50 am this morning. The matter is still being investigated.” said.
The corpse was so decomposed that at first it was mistaken for a doll and therefore was not reported to the police at first.
An autopsy is expected in the coming hours to determine the cause of death.