An alligator attack victim’s son has recounted how she was left with her arm “hanging off by a thread” after the beast pounced while she was swimming in a river.

Dina Spann, 58, was enjoying a day with family boating on the Silver River in Florida’s Marion County when disaster struck on Saturday afternoon.

Her shocked son, Zach Guerrina, who was on the day trip with her, told The Post that bystanders used Spann’s bikini top as a tourniquet to stop the bleeding from her right arm.

“The gator came towards her and she tried to shoo it away with a stick and it attacked her,” Guerrina said. “She got on the bank, a boat pulled up, she got in there with them.

“They tied up the arm with an anchor rope as well, so it had her bathing suit top and an anchor rope on her right arm, and she was just as calm as a f–king cucumber.

“It was a crazy situation and she was the calmest one there, with her arm hanging off by a f–king thread.”