The concept of social mix is perceived as an attempt to minimize social exclusion by enhancing individual social capital. Related policies have been implemented in many countries to prevent isolation of certain groups and achieve social integration. However, few studies have examined their impacts on various income groups. This study uses data from the National Social Capital Survey in South Korea to empirically investigate the impact of the social mix policy on trust, networks, and norms, which are the three types of social capital that promote social integration. Multiple regression models present that the income mix is negatively associated with neighborly trust and networks, while being positively associated with norms. These models also suggest that mixing of income groups improves the norms of low-income groups but reduces the trust of high-income groups. Thus, this study argues for the need to establish specific goals and targets when promoting a social mix policy and understanding the side effects of introducing such a policy.