IMPORTANCE: Social participation is essential throughout life and is associated with decreased mortality and increased quality of life. It is unknown whether long-term care facility (LTCF) entry disrupts or facilitates it. OBJECTIVES: To determine longitudinal trends in social participation before and after entry into nursing homes (NHs) and assisted living facilities (ALs) and to explore factors associated with participation. DESIGN, SETTING, AND PARTICIPANTS: This nationally representative longitudinal cohort study using prospectively collected annual data from the US National Health and Aging Trends Study from 2011 to 2019 included community-dwelling Medicare beneficiaries entering LTCFs. Interviews conducted 4 years before and 2 years after NH or AL entry (index date) were included. Data analysis was performed from September 16, 2022, to May 25, 2025. MAIN OUTCOMES AND MEASURES: Two categories of social participation comprising 5 activities were assessed: socialization (visiting with friends or family and going out for enjoyment) and community participation (attending religious services, participating in clubs or other organized activities, and volunteering). Participation over time was modeled using linear splines before, upon, and after LTCF entry. Modified Poisson regressions were used to explore associations with maintaining and starting activities, adjusted for age, sex, race and ethnicity, and proxy response were used. RESULTS: The total sample included 606 LTCF entrants (weighted mean [SD] age 85 [7.4] years, 404 female [66% weighted]), of whom 104 individuals were Black (7%), 23 Hispanic (4%), 464 White (86%); and 15 of any other race and ethnicity (3%). Before entry, social participation decreased in all activities (-4.7 to -2.0% annually). Of the total, 275 (44%) entered a NH and 331 (56%) entered an AL facility. Upon entry, going out for enjoyment decreased (-14.1%), but club participation and religious attendance increased (15.6% and 12.6%, respectively). Before LTCF entry, social participation decreased in all activities (-4.7 to -2.0% annually). After entry, going out for enjoyment decreased (-14.1%), but club participation and religious attendance (12.6%) increased (15.6% and 12.6%, respectively). In exploratory analyses, women were more likely to maintain visits (adjust risk ratio [aRR], 1.3; 95% CI, 1.1-1.5) and start attending religious services (aRR, 1.6; 95% CI, 1.0-2.8). NH residents were less likely to go out for enjoyment (aRR, 0.6; 95% CI, 0.5-0.8 for maintaining; aRR, 0.6; 95% CI, 0.4-1.0 for starting) and keep attending religious services (aRR, 0.7; 95% CI, 0.6-0.9). Black, Hispanic, and residents of other race or ethnicity were much less likely to start going out for enjoyment (aRR, 0.3; 95% CI, 0.1-0.8). CONCLUSIONS AND RELEVANCE: This cohort study found that LTCF entry generally promoted community participation and reduced socialization. Benefits may be less likely among men, NH entrants, and residents of racial and ethnic minority groups.