A Long Island surfer fought off a shark that chomped down on his leg by “slapping” it and “paddling like hell” to shore, he said Thursday.

Shawn Donnelly, 41, of Mastic Beach — one of five people attacked by sharks on Long Island in the past two weeks — was lying on his surfboard at 7:30 a.m. Wednesday near Smith Point County Park in Shirley when the man-eater sunk its teeth into his left calf, he said.

“The shark ambushed me,” Donnelly told The Post. “I have never been more scared in my entire life … I screamed and flailed.”

Donnelly said the impact of the bite sent him plunging into the 7-foot-deep water roughly 40 feet from shore — where he could see the possible sand tiger shark’s spotted dorsal fin. 

“It came up from the sandbar like a torpedo,” he said. “There was no fish jumping out of the water, no water moving, just quiet then ’bang.’ Next thing I knew the shark knocked me off my board.”

He slapped the apex predator and scrambled back on to his 6-foot board, with the 5-foot shark lurking in the murky water below him.

“I was horrified,” he said. “I just slapped the shark once. I hit it, then instantly turned and paddled for the beach.”

 “When I got back on my board it was under me, so close,” he said.

He then fled for his life — catching a well-timed wave to the coastline.

“I … paddled like hell. After a couple of strokes, a wave broke behind me and pushed me to shore,” he said. 

“I thought, ‘I can’t believe what just happened.’”

Once out of the water, Donnelly saw blood dripping from a 2-inch gash on his leg.

“I did a quick check. I thought maybe he didn’t get me. Then I saw the blood running down my leg and one clear bite mark,” Donnelly told the outlet.

He walked to a nearby campground check-in booth and told a park ranger he’d been bit by a shark. He was then taken to Long Island Community Hospital in Patchogue, where his injury was treated.

“It happened so fast,” he said. “I was afraid when it happened, then when I was back on the beach I had so much adrenalin it didn’t even seem like it was real.”

Donnelly was one of five victims — including a tourist from Arizona and a local lifeguard — who were attacked by sharks in Long Island waters in recent days.

On Wednesday evening, a 49-year-old man from Arizona was standing in waist-deep water on Fire Island’s Seaview Beach when a shark bit him from behind on the buttocks and left wrist, Suffolk County police said.

On July 3, lifeguard Zach Gallo was also chomped in the chest and hand by a shark near Smith Point County Park in Shirley. Four days later, another lifeguard also suffered a shark bite off of Ocean Beach on Fire Island, according to according to ABC News.

On June 30, a 37-year-old swimmer was also bitten on his right foot near Jones Beach.

Donnelly told Newsday he was aware of the recent attacks but assumed he’d be fine.

“It was a calculated risk that I got wrong,” he said. “This summer, whatever fish the sharks are chasing are way closer than normal and we’re bumping into the sharks.”

John Chisholm, an adjunct scientist for the New England Aquarium, said the sand tiger sharks are biting people near Long Island as they hunt for pogies, fish that swim close to the shore during summer months.

“The sharks are not hunting people. It’s just the fact that the people are close to the bait,” he told The Post. “[Sharks are] in more shallow water because that’s where the food is.”

“All along the [shore] from New Jersey to New England, there’s a lot of bait in the water,” he said.

He said the shark encounters are actually a sign that the ocean’s ecosystem is thriving in the area and that beachgoers should adjust accordingly.

“It’s a wilderness area. You wouldn’t go on Safari in the Amazon without planning before you go. So pay attention.”

“Don’t swim or surf by yourself. Don’t go swimming or surfing at dawn or dusk when sharks are more likely to be hunting. 

“Be aware of your surroundings and aware of other animals feeding,” he said.

Long Island waters are also home to spinner sharks, sandbar sharks and juvenile white sharks, which sometimes swim close to the shore in summer months, he said. “You gotta take precautions — and realize the ocean is not a swimming pool.”

Fire Island beachgoers said Thursday the jaws-dropping spate of shark attacks had put a damper on their fun in the sun.

“We’ve been here since Monday. I haven’t swum at all because of the sharks,” said Joy Indursky, 60, who was visiting from Stanford, Conn. 

“I’ve been coming here every year for a gajilion years, but this year was the first year I didn’t swim. I was told it was a little sharky.”

Sara Janzen, 41, who was visiting Seaview Beach from the Upper West Side, wouldn’t let her kids in the water due to the recent shark bites.

“I am a little nervous,” she said. “It was super hot today, and I’m sure my kids would have wanted to go swimming — but I didn’t bring them to the beach.”