Self-control has emerged as a research focus, particularly among adolescents, as they frequently struggle with self-control when studying. We examined the longitudinal relationship between achievement motivation (i.e., attainment value and mastery-approach goal) and self-control at the within-person level after controlling for trait-like interpersonal variance. We used 3-year longitudinal data sets from two panels of the 2010 Korean Children and Youth Panel Survey for the multigroup random intercept cross-lagged panel model. The final analysis included self-reported responses to academic self-control, attainment value, and mastery-approach goal for 3 years from 2,152 early adolescents (11-13 years old) and 2,163 middle adolescents (14-16 years old). Our multigroup random intercept cross-lagged panel models with two different adolescent cohorts revealed strong associations between achievement motivation and self-control at the between-person level, regardless of the adolescent cohort. At the within-person level, early and middle adolescents exhibited distinct longitudinal associations between these two variables. Early adolescents' self-control demonstrated noticeable stability and correlations with mastery-approach goal pursuit. By contrast, middle adolescents' self-control exhibited a fluctuating state, which was predicted by attainment value. Practically, this implies that for early adolescents, self-control may function as a trait that can determine and guide adaptive mastery-approach goal pursuit implicitly and habitually. For middle adolescents, however, self-control may no longer function as a trait; instead, it can fluctuate and be affected by their identity-related attainment value perceived within a given academic context.