The woman was bit while taking a dip in Key Lake Wilderness Park in Cocoa, the Brevard County Fire and Rescue said on Facebook. The department listed it was a "trauma alert," and she was airlifted to a local hospital with "significant bite injuries."
Witness Dave Nygard told ABC News that he initially thought the woman was "joking around" when she started screaming.
"We're hanging out about waist to chest deep in the lake, next thing you know a girl starts screaming and luckily a couple guys reacted and grab her," Nygard said. "... next thing you know we pull her out and her side and her thigh was open. So then about 30 seconds later I see a gator head pop up. It was every bit of 8-foot."
Nygard said there were about a dozen people at the lake and that several of them grabbed towels and shirts to wrap her injuries.
The Florida Fish and Wildlife Conservation Commission described her wounds as "non-life threatening."
Brevard County Fire District Chief Thomas Uzel said attacks in this area are pretty rare.
"As far as alligator bites, I think this was the second in 35 years," he told ABC News.