Objective: We examine concurrent and longitudinal associations between sleep duration (SD) and body mass index (BMI) from adolescence to young adulthood, controlling for physical activity and internalising symptoms. Sex and age group differences (early adolescent group ages 12-15years and late adolescent group ages 16-18years) are examined.Design: A randomly selected community-based sample of youth (N=662; 48.3% males) were surveyed biannually from 2003 (T1) to 2014 (T6).Results: We found sex and age group differences for the concurrent and longitudinal associations between SD and BMI. For males, shorter SD at baseline was concurrently related to higher BMI. For females, shorter SD at baseline was associated with longitudinal increases in BMI, for the late adolescent group only.Conclusion: Findings show that the association between SD and BMI, previously shown in childhood, continues to be significant in adolescence and young adulthood. Developmentally appropriate and sex-specific strategies for motivating and supporting healthy sleep habits in adolescence that may help to prevent weight gain into young adulthood are discussed.