Background The beneficial effects of physical activity (PA) for both physical and mental wellbeing are well established. Given that adolescence presents a critical developmental period during which life-long patterns of PA become established, the exploration of the longitudinal impact of adolescent psychopathology on adult PA status is of interest. Methods We analysed prospective data from 3663 young adults who participated in the Mater-University of Queensland Study of Pregnancy. Psychopathology was measured using the Youth Self-Report (YSR) at age 14. Participants' engagement in three types of PA (vigorous exercise, moderate exercise and walking) at age 21 were dichotomised into either 'none' or 'any'. For our main analysis, we examined the association between the YSR score and subsequent PA engagement using logistic regression. We also conducted sensitivity analyses of longitudinal associations between the YSR internalising and externalising symptoms score at age 14 and PA engagement at age 21. Results We found no longitudinal association between the total YSR score at age 14 and PA engagement at age 21. In addition, there was no longitudinal association between the YSR internalising or externalising symptoms and PA engagement. Conclusion Our findings suggest that there is no longitudinal association between adolescent psychopathology and PA in young adulthood.