That's about 85 million people.
It's the first federal study to look at how often adults eat fast food.
The Centers for Disease Control and Prevention based the study on a survey of about 10,000 adults over four years.
Higher-income families ate fast food more often than lower-income families, and African-Americans ate it more than other racial or ethnic groups.
Health officials say too much high-calorie fast food can lead to obesity, diabetes, heart disease and other health problems.
The Associated Press contributed to this report.