An 11-year-old girl was driving a Can-Am Commander with a 9-year-old girl as passenger when the ATV crashed in Plant City, Fox 13 reported.
The vehicle rolled and the youngest girl was ejected.
The unidentified 9-year-old girl was taken to South Florida Baptist Hospital where she was pronounced dead.
The vehicle is similar to a dune buggy with two-bucket seats, Bay News 9 reported.
Neither girl was wearing a seatbelt, WFLA added.
Plant City is about 25 miles northeast of Tampa.