Background and purpose: Social engagement is an important active aging strategy to promote older adults' mental health. The purposes of this study were to compare social engagement in older populations around the world and explore associations with mental health outcomes. Materials and methods: An international cross‐sectional survey was conducted from 2017 to 2019. Data were retrieved from The International Social Survey Programme for a secondary data analysis across 30 countries. This study applied the Taxonomy of Social Activities and its six levels as operational definitions for a consistent concept of social engagement for international comparisons. Results: In total, 9403 older adults with a mean age of 72.85 ± 6.40 years responded. The highest levels of older adults' social engagement were found in Switzerland, Thailand, and New Zealand. Older adults of a higher age, with a lower educational level, who were permanently sick or disabled, who had no partner, who were widowed or whose civil partner had died, who lived alone, and who had lower self‐placement in society had significantly lower social engagement than did their counterparts. In the regression model, older adults' social engagement positively predicted general health, self‐accomplishment, and life satisfaction, but negatively predicted loneliness and depression. Conclusions: In aging societies worldwide, encouraging older adults' social engagement would be beneficial to promote mental health. Implications for nursing practice and health policies: Community professional nurses can develop strategies of social engagement based on the needs and sociodemographic factors of older adults to improve their mental health. Developing efficient strategies and local policies by learning from successful experiences in other countries is important to promote social engagement in aging societies. [ABSTRACT FROM AUTHOR]