Background Overweight and obesity among young people is alarmingly high. While hundreds of millions of children participate in organised sports worldwide, it is currently unknown whether time spent in organised sports is associated with levels of adiposity among young people. This study aimed to investigate bidirectional associations between participation in organised sports and adiposity over a two year period. Method Data were drawn from the Longitudinal Study of Australian Children. In total, 4033 participants (51% male) reported time spent in organised sports and had their body mass index, body fat percentage, and waist circumference measured at age 12, and again two years later. A cross-lagged panel model was used to examine bidirectional relationships over time, as well as interaction effects. Results Total sport participation at age 12 was not associated with subsequent BMI-z scores (β = 0.01 [95% CI, -0.02, 0.04]), body fat (β = 0.01 [95% CI, -0.02, 0.03]), or waist circumference (β = -0.01 [95% CI, -0.05, 0.02]). Similarly, measure of adiposity at age 12 were not associated with subsequent sports participation (BMI-z score: β = -0.01 [95% CI, -0.02, 0.04]; body fat percentage: β = -0.02 [95% CI, -0.05, 0.02]; waist circumference: β = -0.01 [95% CI, -0.01, 0.03]). There were no differences in the strength or direction of the relationships by type of sport or by sex (p < .05). Conclusion Policy and programmatic changes may be needed before organised youth sports are considered a preventative strategy for overweight and obesity. However, a more nuanced understanding of why organised youth sports are not associated with adiposity is needed before evidence-based changes can be made.