IMPORTANCE Sexual relationships are important for well-being and health. Recent trends in sexual activity among US adults are unknown. OBJECTIVES To examine trends in reported frequency of sexual activity and number of sexual partners by sex and age and the association between measures of sexual activity and sociodemographic variables. DESIGN, SETTING, AND PARTICIPANTS In this survey study, repeat, cross-sectional analyses of participants aged 18 to 44 years from 10 rounds of the General Social Survey (2000-2018), a US nationally representative survey, were performed for men and women separately. MAIN OUTCOMES AND MEASURES Sexual frequency in the past year (sexual inactivity, once or twice per year, 1-3 times per month, or weekly or more) and number of sexual partners in the past year (0, 1, 2, or Ն3 partners). The association between measures of sexual activity and sociodemographic variables were assessed using logistic regression. RESULTS The study population included 4291 men and 5213 women in the analysis of sexual frequency and 4372 men and 5377 women in the analysis of number of sexual partners (mean [SD]