Using the most recent nine waves of data from the General Social Survey, which consists of in-person interviews of independent probability samples of the adult household population of the United States, the purposes of this study were to (a) provide descriptive information on adults’ attitudes toward extramarital sex, lifetime and annual prevalence of extramarital sex among ever-married adults, and the identity of the extramarital sex partner(s) of currently married adults; (b) evaluate temporal trends in attitudes toward and prevalence of extramarital sex from 2000 to 2016; and (c) test for gender differences in attitudes toward and prevalence of extramarital sex and descriptions of the extramarital partner. The percentages of Americans who reported that extramarital sex was always wrong significantly declined from 2000 to 2016, whereas the percentage who reported it was wrong only sometimes significantly increased. There was a statistically significant linear decline in reported lifetime prevalence of extramarital sex from 2000 (17.8%) to 2016 (16.3%), whereas there was no statistically significant change in reported annual prevalence of extramarital sex (3.0%). People most commonly reported having extramarital sex with a close personal friend (53.5%) or neighbor, coworker, or long-term acquaintance (29.4%). Compared with women, men were (a) less likely to report that extramarital sex was always wrong and more likely to view it as almost always wrong, wrong only sometimes, or not wrong at all; (b) more likely to report past-year and lifetime extramarital sex; and (c) more likely to report extramarital sex with someone they knew casually.