Objective The purpose of this study was to examine the mediating effect of social capital on the relationship between Social Media use motives and subjective well-being. Methods In the study, online self-reporting surveys were conducted with Social Media users in their 20s, and data from 445 participants were used for structural equation modeling. Results The main findings of the study were as follows. First, the interpersonal motives for Social Media use had an indirect effect on subjective well-being by mediating offline bonding capital and online bonding capital. In addition, interpersonal motives had an indirect effect on subjective well-being by dual-mediating online and offline bonding capital. Second, the self-expression motive for Social Media use did not directly affect subjective well-being, but it indirectly affected subjective well-being by mediating offline bonding capital. Third, the information-seeking motive for Social Media use did not directly affect subjective well-being, but it indirectly affected subjective well-being by mediating offline bonding capital. Conclusion This study identified a specific mechanism for how motives for using Social Media affect subjective well-being. Furthermore, the results of this study suggest that the effect of Social Media use on subjective well-being may differ depending on the motive for Social Media use.