Background: Little is known about the influence of 24-hour movement behaviors on children’s psychosocial health when transitioning from primary to secondary school. This study described changes in 24-hour domain-specific movement behavior composition and explored their associations with changes in psychosocial health during this transition. Methods: Data were drawn from the Longitudinal Study of Australian Children. The analytical sample (n = 909) included children who were enrolled in primary school at baseline (2010) and in secondary school at follow-up (2012). Time spent in 8 domains of movement behaviors was derived from the child-completed time-use diaries. Psychosocial health was examined using the self-report version of the Strengths and Difficulties Questionnaires. Analyses included repeated-measures multivariate analysis of variance and compositional regression. Results: Children reported engaging in more social activities and sleeping less over the transition period. Increased time spent in social activities (β_ilr = −0.06, P = .014) and recreational screen use (β_ilr = −0.17, P = .003) (relative to other domains) were associated with decreased prosocial behavior in boys. Changes in movement behavior composition were not associated with changes in girls’ psychosocial health. Conclusion: This study found considerable changes in children’s 24-hour movement behavior composition, but a lack of consistent association with changes in psychosocial health during the primary to secondary school transition.