The incident happened around 5:30pm Tuesday.
The girl suffered significant soft tissue damage to her leg and was taken to UTMB. Beachgoers used a dog leash as a tourniquet to stop the bleeding.
"That tourniquet...definitely made an impact on this little girl's condition," said Nathan Jung, Galveston County Health District EMS Administrator.
According to Galveston Island Beach Patrol Chief, Peter Davis, the girl was visiting with her family from Kerrville. She was out in the water in a pink ring tube with her father and older brother. They were among a school of bait fish when she was bitten.
"Our belief is the shark bit the little girl's leg thinking it was a fish and then let go," Davis told Eyewitness News.
He urges beachgoers to avoid swimming or wading near schools of fish.
The father told first responders the shark was 3-4 feet long and had a white belly. Davis says it may have been a blacktip shark which is common in the Gulf of Mexico.
This is the first shark bite off Galveston this year. On average, there are 6 million visitors a year to Galveston and one shark bite every three years, according to beach officials.