Karin Ulrike Stei, 47, of Konstanz, Germany, was bitten by a shark late Wednesday morning north of Humiston Park in Vero Beach, TCPalm.com reported.
Stei and her friend Brigitte Schmid were swimming in waist-deep water about 30 yards from shore when the incident occurred, police said.
Schmid said she didn't realize there was a shark near them until Stei yelled for help.
The shark bit off a large portion of Stei's upper thigh, said lifeguard Erik Toomsoo, who rescued the woman.
Toomsoo was on a nearby boardwalk when he heard the women's yells for help and quickly responded. He pulled Stei to shore, with the aid of several other men who were near the scene.
A fire rescue helicopter was flying over the area when a paramedic on board saw the blood in the water and had the helicopter land to pick up Stei.
Stei was then flown to a Lawnwood Regional Medical Center & Heart Institute in Fort Pierce, police said, where she was in surgery until about 7 p.m. Wednesday.
She is in serious condition, police said.