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Abstract Background Physical activity (PA) participation among youth tends to be insufficient and is prone to decline with age. In Australia, this decline has been shown to particularly occur in the domain of non-organized PA (e.g. active play and informal sport) between childhood and adolescence. However, information about changes in more specific groupings of activities within non-organized PA (i.e. subdomains) is needed, as this could support more targeted intervention strategies. This study aimed to investigate changes in the duration of specific subdomains of non-organized PA between late childhood (10–11 years) and early adolescence (12–13 years) in Australia, as well as whether these changes are moderated by sex. Methods Data were sourced from Waves 6 and 7 of the Longitudinal Study of Australian Children (n = 3614). Youth time-use diaries (24-h) were used to measure the duration of eight subdomains of non-organized PA at both waves (athletics/gymnastics, ball sports, cycling/motor/roller sports, fitness/gym/exercise, martial arts/dancing, water/ice/snow sports, active play and other outdoor/nature PA). Multilevel mixed modelling was used to explore longitudinal changes between waves and the potential moderation effect of sex. Results Active play declined the most of all subdomains (β = –20.5 min/day; 95% CI = –23.4, –17.6, p