Nine girls were thrown off the inflatable attraction when the accident happened at a fair in Mislata, near Valencia on Tuesday night.
Panicking parents watched as the castle was thrown into the air and collapsed on top of their children in the ground nearby.
Emergency workers joined parents, police and fair workers to free the girls from the wreckage.
The girl who died was thrown into the air and landed on her head.
She died from her injuries on Wednesday in La Fe Hospital in Valencia about 12 hours after the accident.
“One of those injured has sadly died,” Valencia health authorities said in a statement.
Another four-year-old girl was in a serious condition in hospital with multiple injuries.
A police officer at the scene was able to stem the bleeding of the girl’s injury until emergency workers arrived to give medical aid.
Seven other girls suffered less serious injuries and received hospital treatment.
A gale of up to 72kmh sent the inflatable castle flying away from its moorings just after 8pm on Tuesday, throwing the girls on to the ground nearby.
All were taken to nearby hospitals in the Valencia region.
Police have launched an investigation into the tragedy to clarify if the company which staged the fair had any responsibility.
The bouncy castle had passed an annual inspection and another check by a qualified engineer, according to a report in Las Provincias, a local newspaper.
One line of investigation for the police will be to see if the bouncy castle was sufficiently anchored to the ground before the high winds last night.
Carlos Fernandez, mayor of Mislata, a town of 43,000 inhabitants on the outskirts of Valencia, said: “The police will determine what happened.”
Mislata council said in a statement: “Following the tragic incident last night, we have declared a period of official mourning until 7 January because of the death of one of the girls involved.
“We would like to transmit our sincere condolences to the family and friends of the youngster who lost her life. She was only eight years old.”
Mislata council has suspended all events organised for the traditional procession of the Three Kings on Wednesday night.
Across Spain, Melchior, Caspar and Balthazar traditionally arrive in cities and towns on the eve of Epiphany tomorrow when Spanish children receive their Christmas presents.
A similar tragedy happened in Australia last month when six children lost their lives.
Nine students fell 32 feet from an inflatable castle when it was lifted up by wind at a school fair in Devonport, in Tasmania. All were aged between 11 and 12.
Chace Harrison was the sixth child to die from his injuries after his life support machine was switched off in hospital on 19 December.
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