A local Hawaii news outlet reported that the 25-year-old victim was alongside his father Saturday when a 12-foot tiger shark knocked both of them into the water.
State Department of Land and Natural Resources Dan Dennison said in an email alert that the incident happened around 9:30 a.m. Saturday on Kukio Beach near Hualalai.
Ocean safety crews distracted the shark with a jet ski, while good Samaritans and an off-duty firefighter applied tourniquets to the victim.
Officials say the victim is missing part of his leg and has arm injuries. His father was not hurt.
The Associated Press contributed to this report.