The woman was snorkeling in Makena when witnesses said they heard her scream about 4:40 p.m., local time, Wednesday, HawaiiNewsNow.com reported.
"We heard screaming from the water and it was this unbelievable scream like I've never heard before," witness Andree Conley-Kapoi said.
Two friends and a kayaker helped the woman to shore where she stopped breathing following a massive blood loss, KHON-TV, Honolulu, reported. They performed CPR on her until emergency responders arrived.
Beaches a mile in each direction of the attack were closed and were expected to remain closed for at least 24 hours after the attack, HawaiiNewsNow.com said.
This marked the eighth shark attack in Hawaii this year, KHON said.