Maui County said Friday the man was snorkeling with another man about 200 yards off Kihei when the shark attacked.
The county says the man suffered multiple cuts on his lower left leg and thigh.
He told authorities the shark's head was about two feet wide, but it's not known what type of shark attacked him.
Lifeguards brought him to shore and treated him. He was then taken to Maui Memorial Medical Center.
The beach from Kamaole III to Waipuilani Park is now closed to swimmers.