Her doctors warned that another sting could be life-threatening.
The 62-year-old had been in the water for more than 30 hours and was hoping reach Florida Monday morning.
Nyad tried this marathon swim in August, but she had to quit after a serious asthma attack made it difficult to breathe.
Swimming from Cuba to Florida is a lifelong dream that Nyad first tried as a 28-year-old back in 1978, when she swam inside a steel shark cage for about 42 hours before ending the attempt.
This time she was trying to do it without a cage, relying on electronic equipment to deter sharks.
The Associated Press contributed to this story.
