Recent evidence underscores the detrimental impact of affective polarization on democracy, making it imperative to understand its underlying causes. This article explores the role of social capital in mitigating affective polarization. We argue that social capital fosters connections across diverse societal groups and promotes pluralism and tolerance for different worldviews, thereby reducing affective polarization. To test this hypothesis, we employ two empirical approaches. At the individual level, we examine whether citizens with greater civic engagement, measured through associational membership and volunteering, are less affectively polarized using longitudinal data from the Dutch LISS panel (2007–2024). At the macro level, we assess whether countries with higher social capital are less affectively polarized, leveraging data from the Comparative Study of Electoral Systems (CSES) and original smooth country-year panels of social capital, estimated by combining data from multiple international surveys. Our findings will have crucial implications for safeguarding democracy in polarized societies.