<p>Psychological flexibility is crucial in adolescent mental health by supporting effective stress management. The present study aimed to investigate the mediating role of sense of coherence in the relationship between psychological flexibility and positive youth development. Study I (<i>N</i> = 391; 58.1% girls; <i>M</i>_age = 15.24, <i>SD</i> = 1.03) employed a cross-sectional design and was analysed solely at the between-person level. Study II (<i>N</i> = 102; 59.8% girls; <i>M</i>_age = 15.88, <i>SD</i> = 1.18) utilised a daily diary design and was analysed across four waves, incorporating both within-person and between-person levels. The findings of this study demonstrated that psychological flexibility is positively associated with sense of coherence and positive youth development. Furthermore, the simple mediation analysis in Study I and the multilevel mediation analysis in Study II indicated that sense of coherence mediates the relationship between psychological flexibility and positive youth development. These findings indicate that psychological flexibility enhances sense of coherence, which in turn supports positive youth development. These results suggest that strategies grounded in psychological flexibility enhance sense of coherence, thereby fostering positive youth development.</p>