The incident happened Sunday morning about 100 yards from the shore. City officials say the victim, a 46-year-old man named Caleb, was in the water with a group of about a dozen swimmers at the time.
In an exclusive interview with Eyewitness News, swimmers Cameron Whiting and Kevin Barrett said instinct kicked in when they heard his screams for help.
"We heard some screaming and you could tell it was serious from just the tone in the screams," Whiting said. "You have to realize that you're swimming into an active shark attack and it's scary, but there's not really an alternative and he's one of our own and we have to go bring him in."
Though it's unclear what type of shark attacked Caleb, the swimmers say juvenile great white sharks are the biggest threat in that area.
They noticed he had been bitten in the chest and later learned that he punched the shark in an effort to escape.
"He caught the shark on the tooth so he had a pretty big gash in his hand that was bleeding profusely as well," Barret said.
"We didn't know the extent of the injuries, but there was definitely a lot of blood in the water and so we started pulling him in, a surfer came over and gave us his board," said Whiting. "(I) put him on the board and I jumped on top of him and started paddling in."
The lifeguards weren't even on duty yet, they happened to arrive early and helped the swimmers and called an ambulance.
"There was actually an E.R. doctor who happened to be walking by on the beach as well, so he came and he helped out, too. I think all stars were aligning to help Caleb through this," Barret said.
The victim was taken to a local hospital with non-life-threatening injuries.
A water closure will remain in effect until at least Tuesday morning, Del Mar city officials said.
This shark attack comes a week after a surfer was knocked off his board by his shark off San Clemente.
Shark encounter forces closure of waters at all San Clemente beaches