Objective: This study examined associations between objective and subjective sleep parameters, and mental health symptoms (internalizing symptoms, externalizing symptoms, and irritability) and whether these relationships differed for adolescents with and without ADHD. Method: Participants were 373 adolescents aged 10 to 19 years ( M = 13.21, SD = 2.37). Sleep was measured both via actigraphy and self- and parent-report scales. Mental health symptoms were measured via a range of validated measures. Results: Few relationships were detected when examining actigraphy variables. Subjective sleep parameters were associated with almost all mental health variables. Some relationships were moderated by ADHD status, generally consistent with stronger relationships between subjective sleep quality and mental health in the non-ADHD group. Conclusion: Sleep problems, particularly subjective difficulties, are associated with a range of mental health outcomes, regardless of the presence of ADHD. Sleep problems may be a beneficial intervention target for adolescents experiencing mental health issues.