Back pain causes enormous financial and disability burden worldwide, which could potentially be reduced by understanding its determinants to develop effective prevention strategies. Our aim was to identify whether modifiable risk factors, weight and physical activity, are predictive of back pain in young adult women. Women born between 1973 and 1978 were randomly selected from the national health insurance scheme database to participate in The Australian Longitudinal Study of Women's Health. Self-reported data on back pain in the last 12 months, weight, height, age, education status, physical activity, and depression were collected in 2000, 2003, 2006, 2009, and 2012. In 2000, 9688 women completed the questionnaire and 83% completed follow-up 12 years later. At baseline, median age was 24.6 years and 41% had self-reported back pain. For every 5 kg higher weight at baseline, there was a 5% (95% confidence interval [CI] 4%-6%) increased risk of back pain over the next 12 years. Higher weight at each survey also predicted back pain risk 3 years later (P<0.001). The effects of weight on back pain were most significant in those with BMI ≥25 kg/m2 and were observed at all levels of physical activity. Inadequate physical activity and depression were independent predictors of back pain over the following 12 years (both P<0.001), after adjusting for age, weight, height, and education status. Back pain is common in community-based young adult women. Higher weight, inadequate levels of physical activity, and depression were all independent predictors of back pain over the following decade. Furthermore, the adverse effects of weight on back pain were not mitigated by physical activity. Our findings highlight the role of both higher weight and physical inactivity in back pain among young women and suggest potential opportunities for future prevention.