The girl was identified by an acquaintance as Sage St. Clair, a 16-year-old student at Granada High School.
"My husband felt something hit his back, it went quickly, I heard Sage scream," said Sage's mother, Kim St. Clair.
St. Clair says her daughter and husband were swimming not far from shore about 10 to 15 feet in water only three to four feet deep near the Hololani resort when the shark attacked.
"Sage gets out of the water and she says something bit me and I looked at her leg and she had a bite. I wrapped it up in a towel and called 9-1-1.," St. Clair said.
The gash on her lower left calf is about three to five inches deep.
Unprovoked shark attacks are rare in Hawaii. Several sources including the International Shark Attack File, report there have been 105 attacks since 1828, eight of then deadly.
"You tell the kids 100 times don't worry about the sharks, you're going to be fine. I guess I can't really say that anymore," St. Clair said.
According to the Fire Department, the incident took place in the ocean fronting the Hololani Resort, 4401 Lower Hanoapiilani Highway at 9:52 a.m.
Fire crews treated and dressed Sage's wound before the girl's family took her in their own vehicle to seek further treatment, according to the fire department.
County and state officials treated the incident as a shark attack and closed the beach until 6:45 p.m. today.