Background <p>We examined the role of psychosocial factors in overweight and obesity development.</p> Methods <p>UK Millennium Cohort Study data of children with normal weight at baseline&#xa0;were analysed. Weight changes were determined from baselines at ages 11 (<i>n</i> = 7979) and 14 (<i>n</i> = 6906) to follow-up at age 17. Baseline individual psychosocial factors were combined into two distinct indexes (caregiver-reported child mental health, child-reported psychosocial well-being). Regression models examined the associations between baseline indexes and individual psychosocial factors and overweight and obesity development (vs. no&#xa0;development) and body mass index (BMI) z-score changes.</p> Results <p>Worse child mental health, but not psychosocial well-being, at age 11 was associated with overweight and obesity development (OR = 1.14;&#xa0;95%&#xa0;CI = 1.02, 1.27) and increased BMI z-scores (<i>β</i> = 0.08;&#xa0;95%&#xa0;CI = 0.04, 0.12) to age 17. No psychosocial indexes at age 14 predicted the outcomes. Further analyses showed that child mental health at ages 11 vs. 14 was more likely to predict the outcomes. Based on individual factors, externalising symptoms and experiencing peer bullying at age 11 may be important contributors to overweight and obesity development.</p> Conclusions <p>Poor child mental health at age 11 is associated with overweight and obesity development by age 17. Late childhood/early adolescence may be a sensitive period in which psychosocial factors predict body weight trajectories.</p> Impact <p><UnorderedList Mark="Bullet"> <ItemContent> <p>Worse psychosocial factors, particularly poor mental health, at ages 11, but not 14, were associated with overweight and obesity development and increased BMI z-scores by age 17.</p> </ItemContent> <ItemContent> <p>Late childhood/early adolescence may be a sensitive period for mental health in predicting future weight change.</p> </ItemContent> <ItemContent> <p>Future research will benefit from exploring this potential sensitive period and understanding potential mechanisms.</p> </ItemContent> </UnorderedList></p>