The Maui Fire Department said the incident happened around 9 a.m. at Honokowai Point near Ka'anapali Beach Park area on Maui as the man was swimming about 60 yards from shore.
The 65-year-old man was pulled ashore with injuries that were "traumatic, life-threatening" and consistent with a shark attack, fire officials told KHON2.
Allison Keller, who witnessed the man being pulled to shore, told Hawaii News Now the man looked unconscious as he was transferred to a gurney.
“As we got closer, I saw some blood on his stomach and then I got looking a little bit more and his wrist, it looked like the skin on his wrist was just torn off," Keller told Hawaii News Now. "And then I got looking closer and his entire left leg from his knee down was just missing. There was no blood or anything.”
Maui officials said the man was swimming in water roughly 20 to 25 feet deep. Winds at the time were light, with 1-foot surf and clear waters at the time.
The man's name has not been released, but Keller said he was vacationing on Maui with his wife.
Saturday's attack was the sixth shark attack in Hawaii so far this year, according to Hawaii's Department of Land and Natural Resources Division of Aquatic Resources.
There were three shark attacks in 2018, and out of more than 120 attacks on record since 1995 the incident on Saturday was only the fifth fatal attack in the islands on record, according to KHON2. All five of those deadly attacks occurred on Maui.
Officials have not said what type of shark was involved in the attack, but shark expert Michael Domier told KHON2 a tiger shark could be to blame.
"Maui has for some reason been a hot spot in the Hawaiian islands for tiger shark attacks, and I don't think anybody can really say with any certainty why that is," he told the television station. "There's certainly great tiger shark habitat on Maui, a lot of shallow water, but a lot of people in the water, too."
The last fatal attack happened on April 29, 2015, when the victim was snorkeling in Ahihi Bay, Kanahena Point, Maui.
Authorities said shark warning signs were being posted in the Ka'anapali Beach Park area on Maui, where the man died and will remain in place until at least noon Sunday.
The Associated Press contributed to this report.