Maui County emergency responders say the woman's right arm was severed as she snorkeled about 50 yards off shore at Palauea Beach.
The woman, who is about 20 years old, was helped to shore by two male friends and a kayaker and rushed to Maui Memorial Medical Center. A hospital spokeswoman did not immediately respond to phone messages left before office hours Thursday morning.
The Honolulu Star-Advertiser reports the attack was the fifth off Maui this year and the eighth in Hawaii, according to state Department of Land and Natural Resources statistics. Last year saw 11 attacks, the most in a dozen years and far above the annual average of 4.75.
The woman was snorkeling at a section of the beach known as White Rock. There was choppy water and limited visibility.
"She screamed and called for help," said Maui Fire Services Chief Lee Mainaga.
The Maui Fire Department took a call on the emergency at 4:41 p.m.
The woman was unconscious when she reached the beach on the kayak.
"Her right arm was severed right below the shoulder," Mainaga said. The limb was not found, he said.
The woman's friends did not see the attack and Mainaga said it is assumed, but not confirmed, that she was injured by a shark.
County officials closed beaches for a mile in both directions.
The fire department and lifeguards flew in a helicopter along the coast to look for sharks, the Land and Resources Department said in an email announcement.
Hawaii has not seen a fatal shark attack since April 2004. Willis McInnis, 57, died in an attack as he surfed off Maui's Pohaku Park. The attack left him with a 14-inch bite wound in his right thigh and he died from a loss of blood.