The common dolphin, which suffered wounds near its tail, died at about 9:30 a.m. A necropsy will be performed to determine what caused the dolphin's death.
The dolphin was not swimming well in the tank at the Riverhead Marine Foundation, where it was taken after the rescue.
But rescuers were concerned after the dolphin sank several times to the bottom of the tank.
As rescue workers attempted to draw blood to find out what was wrong, the dolphin died on their stretcher.
The dolphin, nearly 8 feet long and weighing about 300 pounds, was bleeding and struggling when found on the beach Thursday afternoon.
It was not immediately clear what caused the dolphin's initial wounds.