This study examined the association and directionality of effect between mental wellbeing and depressive symptoms in Australian adolescents. Data were collected on two occasions 21 months apart. At Time 1, 1,762 10- to 14-year-old adolescents from a range of socio-economic status areas participated. At Time 2 (T2), 1,575 participated again. On both occasions, the Short Warwick-Edinburgh Mental Wellbeing Scale (SWEMWBS) and the Children's Depression Inventory 2 (CDI 2) were administered via online survey. Cross-lagged, longitudinal path analyses demonstrated a negative association between earlier symptoms of depression and later positive mental wellbeing, and that the reverse was also true, though weaker. The model accounted for 20% of the variance in males' T2 CDI 2 depressive symptom scores (26% for females) and 21% of the variance in males' T2 SWEMWBS mental wellbeing scores (23% for females). Depressive symptomatology and mental wellbeing were highly correlated, but symptoms of depression were more strongly associated with later mental wellbeing than vice versa. This has implications for educational psychologists, teachers, health professionals, and policy makers seeking to reduce depressive symptoms or promote mental wellbeing. Focusing solely on the promotion of mental wellbeing, without intervening to reduce symptoms of depression, may limit the potential outcomes that might be achieved.