Abstract Disclosure: R. Whooten: None. S. Rifas-Shiman: None. W. Perng: None. J. Chavarro: None. E. Oken: None. M. Hivert: None. Purpose: Adiposity is a risk factor for polycystic ovary syndrome (PCOS). Limited knowledge exists regarding modifiable behaviors, such as physical activity, during early-life may mitigate the association of adiposity with PCOS risk. We aimed to assess how moderate-vigorous physical activity (MVPA) may impact the association of adiposity in early adolescence with later report of adolescent PCOS. Methods: Within the Project Viva prospective cohort, we used multivariable logistic regression to assess the association of body mass index z-score (BMIz) at early adolescence (mean age 13.1 years) with mid-late adolescent PCOS (self-reported diagnosis or oligo-anovulation with clinical/biochemical hyperandrogenism at mean age 17.7 years). We stratified our analyses by actigraphy-assessed MVPA during early adolescence (≥30 versus <30 minutes/day). In multivariable models, we adjusted for maternal education and PCOS; and child race and ethnicity, dietary behaviors, actigraphy-assessed sleep duration, and self-reported screentime; and season of actigraphy assessment. Results: Among 341 female adolescents, n=47 (14%) met criteria for PCOS at mid-late adolescence. At early adolescence, mean (SD) BMIz was 0.32 (1.10) units and n=110 (32%) had ≥30 minutes/day MVPA. There was no difference in MVPA ≥30 minutes/day among those with versus without later PCOS (30% vs 33%, p=0.70). In unadjusted analyses, there was an association of early adolescent BMIz with mid-late adolescent PCOS (OR=1.93, 95%CI 1.39, 2.69 per unit increase in BMIz). In stratified analyses, the association of early adolescent adiposity with later PCOS was stronger in those with MVPA <30 minutes/day (OR=2.07, 95%CI 1.37, 3.11) than in those with MVPA ≥30 minutes/day (OR=1.72, 95%CI 0.97, 3.06). These findings persisted after adjusting for all covariates (overall: OR=1.72 95%CI 1.15, 2.56; MVPA <30 min/day: OR=2.01 95%CI 1.17, 3.45; MVPA ≥30 minutes/day: OR=1.36 95%CI 0.64, 2.91), however p-values for interaction terms were non-significant. Conclusion: Our findings suggest that maintaining ≥30 minutes MVPA daily during early adolescence may attenuate the association of adiposity with risk of PCOS by late adolescence. This has implications for the importance of promoting physical activity among at-risk individuals. Presentation: 6/2/2024