The woman, believed to be in her 60s, suffered injuries to her foot and calf and was taken to Maui Memorial Medical Center. The injuries were not life threatening, said Maui County spokeswoman Mahina Martin.
Authorities did not immediately release the woman's name or hometown.
The attack, which occurred off Keawakapu Beach in Kihei,t was reported at 8:34 a.m. by a bystander on the beach. The size and type of shark was not immediately known.
At 7:30 a.m., a surfer reported that his friend's surfboard had been bumped by what appeared to be a tiger shark at nearby Kamaole Beach Park II, prompting a closure of that beach and a shark alert by the county.
"Normally, what happens in a shark sighting is they close that front area and a one mile (1.6 kilometers) up and down the beach," Martin said.
She said Keawakapu is more than one mile from Kamaole.
After the attack, the beach closure was expanded to a four-mile (6.4-kilometer) stretch from Kalama Park to Wailea. The area was being monitored by lifeguards and state wildlife officials.
The last shark attack in Hawaii occurred in November in the same area. Kyle Gruen, 29, was bitten by a shark he estimated to be six to 10 feet (1.8 to 3 meters) long while swimming off Kamaole. Gruen, of Vancouver, Canada, suffered injuries to his hand and leg.
It was one of four Hawaii shark attacks in 2006. None were fatal.
There are about 40 species of sharks that live in Hawaiian waters, but the most frequently encountered are the tiger, whitetip reef, sandbar, and scalloped hammerhead sharks, according to the state Department of Land and Natural Resources.