And the study suggests same-sex encounters between women are almost twice as common as between men.
Researchers found that among 'Millennials' - adults between the ages of 18 and 29 during the 2010s - one in eight women (12.2 per cent) and just under one in 12 men (7.5 per cent) reported having had a same-sex experience.
The study also suggests that acceptance of same-sex sexuality has increased among all generations, with Millennials being the most accepting.
Researchers analysed data from the General Social Survey, a nationally representative survey of more than 30,000 adults that asked Americans about their attitudes toward same-sex behaviour since 1973 and about sexual partners since 1989.
Between 1990 and 2014 the percentage of men who reported having had sex with at least one man increased from 4.5 per cent to 8.2 per cent.
The number of women reporting having had sex with at least one woman increased from 3.6 per cent to 8.7 per cent.
The percentage of adults reporting having had sex with both men and women rose from 3.1 per cent to 7.7 per cent.
Among Millennials, adults between the ages of 18 and 29 during the 2010s, 7.5 per cent of men and 12.2 per cent of women reported having had a same-sex experience.
Lesbian sexual experiences are more likely to occur when women are young, the researchers found, while youth doesn't appear to be a factor for male gay sexual experiences.
In terms of attitudes towards same-sex relations, between 1973 and 1990, the percentage of adults who believed "sexual relations between two adults of the same sex is not wrong at all" rose only slightly, from 11 per cent to 13 per cent.
Since then, however, such acceptance rose to 49 per cent in all adults and 63 per cent of Millennials in 2014.
Study author Professor Jean Twenge, of San Diego State University, said: "These large shifts in both attitudes and behaviour occurred over just 25 years, suggesting rapid cultural change."
Prof Twenge said several social and media factors are driving this change, but broadly Americans appear to care less about social norms and more about their own wants.
She added: "These trends are another piece of evidence that American culture has become more individualistic and more focused on the self and on equality.
"Without the strict social rules common in the past, Americans now feel more free to have sexual experiences they desire."
The findings were published in the journal Archives of Sexual Behaviour.