<p>The associations among trait anxiety, dispositional hope, and mental well-being have been examined in cross-sectional studies. However, no longitudinal research has examined the relationships between these variables, creating a significant gap in the literature. This study collected data at two-time points (T1–T2) at six-month intervals. The study’s participants were 297 adolescents. This study employed a longitudinal design to investigate whether trait anxiety (T2) and dispositional hope (T2) mediate the association between trait anxiety (T1) and mental well-being (T2) in a sample of Turkish adolescents. A cross-lagged panel model was used in this study for a half-longitudinal design to test mediation. The results showed that dispositional hope (T2) had a longitudinal mediating role in the relationship between trait anxiety (T1) and mental well-being (T2). Additionally, it was found that trait anxiety (T2) had a longitudinal mediating role in the relationship between trait anxiety (T1) and mental well-being (T2). Based on the findings of this study, mental health professionals may utilize hope-oriented interventions in the prevention of trait anxiety among adolescents.</p>