AbstractYoung people are worried about climate change but the association with current and past mental health symptoms is rarely examined in longitudinal population-based samples. Drawing on a population-based birth cohort from the Canadian province of Quebec (n = 1325), this study used a cross-over design to (1) test the association between climate change worry at age 23-years and concurrent mental health symptoms assessed on standardised instruments, and (2) test the association between adolescent (15 and 17 years) symptoms of anxiety, depression, inattention-hyperactivity, and aggression-opposition and climate worry at age 23-years. Participant sex, cognitive ability, socioeconomic status, and parental mental health were adjusted for. Descriptive statistics showed that most participants were worried about climate change: 190 (14.3%) were extremely worried, 383 (28.9%) were very worried, 553 (41.7%) were somewhat worried, and 199 (15.0%) were not at all worried. In analysis 1, worry about climate change was associated with significantly higher concurrent anxiety, depression, and self-harm symptoms, even after adjustment for adolescent symptoms. In analysis 2, anxious adolescents were significantly more likely to be extremely worried about climate change six years later (RRR = 1.51, 95%CI = 1.10–2.07), while aggressive-oppositional adolescents were significantly less likely to be somewhat worried (RRR = 0.79, 95%CI = 0.63–0.0.99), very worried (RRR = 0.61, 95%CI = 0.48–0.78), or extremely worried (RRR = 0.51, 95%CI = 0.37–0.72). Taken together, participants who were worried about climate change had more concurrent mental health symptoms but were also more likely to have prior symptoms. Adolescents with higher anxiety were more likely to worry about climate change in early adulthood, while those with higher aggression-opposition were less likely to worry. Future studies should track climate worry longitudinally alongside symptoms using prospective follow-up studies.