The girl fell between 7.5m and 15m from the interior atrium to a lower deck of the Carnival Glory vessel on Saturday morning.
The ship, operated by Carnival Cruise Lines, had recently arrived in Port Miami after finishing a voyage in the Caribbean.
The tragedy took place as passengers were waiting to disembark, it was reported.
Witness Raymond Kassim, told 7News Miami he heard someone cry out.
‘You heard, “Oh, my baby, oh, my baby”, and when you looked, she was down at the bottom,’ he said.
‘It happened so fast. She just fell over.’
Emergency staff on the ship tried to save the girl’s life after the fall.
Emergency medical technician Bob Dorr, who was among the first on the scene, said: ‘She had severe injuries.
‘I started doing CPR until the doctor showed up, and then the doctor took over.’
The girl was taken to Ryder Trauma Center three miles away from the port in a critical condition, but she died there later.
She has not been named, but her family are understood to be from the Bahamas and had just finished a seven-day cruise.
A relative said she was only out of her family’s sight for a few seconds before she fell.
In a statement, Carnival Cruise Lines said: ‘Our most heartfelt condolences are with the family at this very difficult time.’
Miami Fire Rescue captain Ignatius Carroll said the distance the girl fell has not been determined, but could have been between 25 and 50 feet.
Police are investigating the incident.
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