To explore the mechanism between personal belief in a just world and mental wellbeing, this study focused on the mediating effects of gratitude and prosocial behavior toward friends/strangers. For this purpose, self-reported data of 154 college students in Seoul, Chungnam, and Jeonnam were analyzed. As a result, personal belief in a just world positively predicted gratitude and mental wellbeing. The mediating effect of gratitude was also significant. Furthermore, the double mediation effect of gratitude and prosocial behavior toward strangers was significant. However, neither the mediating effect of prosocial behavior toward friends/strangers, nor the double mediation effect of gratitude and prosocial behavior toward friends was significant. This study is of note in that it verified the mechanism by which personal belief in a just world predicts mental wellbeing. In particular, it was interesting to see that prosocial behavior has different effects on mental wellbeing depending on its targets. Implications for intervention to improve mental wellbeing of college students were discussed.