AbstractAlthough identity research has predominantly focused on college-bound adolescents, it has largely neglected vocational high school students who enter the job market immediately after graduation. Furthermore, most studies have concentrated on personal identity and have overlooked the concurrent development of social identity. This study examined the relationship between adolescents’ personal and social identities over three years of vocational high school. The participants were 4,264 vocational high school students in Japan (Time 1: 46.44% girls; Mage = 15.78). Using a random intercept cross-lagged panel model, the results indicated that within-person increases in social identity predicted increases in personal identity one year later. These findings highlight the importance of social identity as a valuable resource for personal identity development among vocational high school students, a group underrepresented in identity research.