The 61-year-old was trying to set a world record by swimming from Havana, Cuba to Key West, Florida without a shark cage.
She was in the water for about 29 hours before wind conditions, asthma and shoulder pain got the better of her. The swim was expected to cover 103 miles in 60 hours.
"It felt like this was my moment," a quote attributed to Nyad on her /*Twitter*/ feed said. "I don't feel like a failure at all. But we needed a little more luck."
According to the Twitter feed, Nyad was on a support boat after ending the swim and wrapped in blankets. The boat arrived at Key West early Tuesday.
If Nyad had finished, she would have broken her own 1979 record of 103 miles for a cageless swim.
The Associated Press contributed to this story.