The purpose of this study is to examine the association between social capital and mental health using multilevel analysis to fulfill some limitations of previous studies. A cross-sectional survey was conducted in Seoul, South Korea. The final sample comprised 4,734 respondents. The results show that some dimensions of social capital are associated with mental health at the individual level, but the contextual-level social capital variables are not. Our results suggest that both individual-level trust and organizational participation are positively associated with mental health. Based on the findings, decision makers may need to consider designing policies that leverage individual-level social capital for the enhancement of citizens’ mental health. In addition, it may not be cost-effective if social capital interventions targeting mental health are focused only on the contextual level.