This study examined the longitudinal relationships among self-concept clarity (SCC), hope, and subjective well-being (i.e., emotional well-being and cognitive well-being). Specifically, we tested both the between-person and within-person associations of SCC with subjective well-being among 2,001 Chinese adolescents (age range 11-24 years, 42.9% males) during the first year. In addition, we examined the longitudinal mediating role of hope. Results showed that SCC had positive correlations with hope and subjective well-being both at one time point and over time. Multilevel mediation analysis indicated that hope could serve as a mediator in the association between SCC and subjective well-being, at both the between-person and within-person levels. Moreover, the model of longitudinal multilevel mediation was found to be moderated by gender and age. Our results indicated that hope could be an underlying mechanism for the longitudinal relationship between SCC and subjective well-being among Chinese adolescents, which provides a potential intervention target for improving adolescents' well-being.