The 61-year-old swimmer had reached roughly the halfway point of her journey.
Veteran swimmer Diana Nyad had been attempting a 103-mile-long swim from Cuba to Florida.
The 61-year-old had to abort the attempt early Tuesday morning after 29 hours of swimming.
Reports said she had encountered strong winds and had been vomiting when she was finally pulled from the water.
Marathon swimmer Diana Nyad announced early Tuesday that she was forced to end her second attempt to swim from Cuba to the Florida Keys.
"It's over," said a short message sent over Nyad's Twitter feed to the thousands of people following the 61-year-old's record attempt.
After reaching just short of the halfway point of her some 103-mile swim, Nyad encountered a "combination of factors" that proved to be too much to safely continue, according to an explanation posted on Twitter. She was about 29 hours into the swim. Twitter CNN, which had been tracking Nyad's swim via GPS, reported she was vomiting when she was pulled out of the water at about 12:45 a.m. The network said ocean swells, shoulder pain and asthma forced her to quit.