This study aimed to analyze the mediating effect of grit in the relationship between basic psychological needs and subjective well-being among Korean and Chinese adolescents. The study subjects were a total of 300 Korean and Chinese adolescents. As a result of the study, first, the Korean adolescents showed a high level of autonomy in basic psychological needs, while Chinese adolescents showed a high level of competence. Second, there was a significant positive correlation between basic psychological needs and grit as well as subjective well-being. Third, we established that the direct effect of basic psychological needs on subjective well-being was more potent in Korean adolescents than Chinese adolescents. Fourth, the indirect effect of basic psychological needs on subjective well-being through grit was statistically significant. Fifth, the mediating effect of grit was found to be stronger among Chinese youth than Korean youth. Based on these findings, we discussed the importance of grit to improve the subjective well-being of adolescents, and the approaches to improve Korean youth's competence and Chinese youth's autonomy.