Bethany Hamilton, 13, had been lying on her board in clear blue waters off Kauai's North Shore in Hawaii when the shark struck. Its bite also took a huge chunk of her board, but it struck only once, ignoring her companions and disappearing back into the ocean.
"My arm was hanging in the water, and it just came and bit me," she told a Honolulu television station. The shark yanked her back and forth, she said, "but I just held on my board, and then it let go."
"Nobody saw it happen," her father said. "She just yelled, 'A shark bit me'."
Doctors at the Wilcox Memorial hospital said yesterday that being an athlete had helped her to survive the attack last Friday. "It was a very clean amputation," said one. "This is a highly trained athlete, and because of that she's able to handle a huge blood loss."
She had been surfing with best friend, Alana Blanchard, also 13, and Alana's father. He applied a tourniquet to what was left of her arm, using a surfboard leash.
The water was clear and there had been no sign of a shark nearby, witnesses said.
The chunk bitten from the board suggests that the shark was up to 4.6 metres long (15ft) and could have been a tiger shark.
Ms Hamilton had hoped to go professional. "She is probably the best young surfer I have ever seen," said Bobo Bollin, whose company sponsors a team of which she is a member.
Mr Hamilton said he had asked her whether she would surf again. "She's back and forth," he said.