This study investigates some of the key social factors associated with mental health by focusing on the role of social capital defined in terms of two related yet distinct variables: general trust and social support (from kin and non-kin members). Data from the 2010 Korean General Social Survey (KGSS), which consists of a nationally representative sample, are analyzed to empirically examine the associations between the two social capital measures and self-rated mental health. Given the clustered structure of the data, multilevel or hierarchical linear models are estimated. While controlling for a host of socioeconomic and other background variables, individuals who have greater trust in generalized others (i.e., strangers) and those who receive more support from non-family members report themselves as being mentally healthier. Help from kin, on the other hand, has no significant effect. The current research suggests that there are critical social determinants of health, which are important for improving and maintaining mental well-being for the Korean adult population.