The man was spearfishing off Jupiter Inlet when he was bitten, according to ABC affiliate WPBF-TV in West Palm Beach.
A witness told WPBF he saw the victim when he was being brought back to shore.
"He was bleeding quite a bit," said Peter Pinello. "It looks like a shark had gotten him (in the torso) You could see his wetsuit was embedded in it, and it looked like it took a good chunk of it out."
Pinello told WPBF he heard others say they believed a bull shark had bitten the man.
Palm Beach County Fire Rescue Battalion Chief John Vanek told WPBF that the victim was bitten on the head but was conscious when he was flown to the hospital. An update on his condition wasn't immediately available.