The purpose of this study was to longitudinally examine the effects of parent and peer motivational climate on sport autonomy motivation in early adolescent athletes. We hypothesized that parents’performance-oriented motivational climate would negatively affect player autonomy, but peers’mastery-oriented motivational climate would moderate negative effects on player autonomy. Participants were 74 soccer players (66 male, 8 female) who completed the study at two time points separated by one year, with a mean age of 11.04 years (SD=0.71 years) at the first time point. At Time 1, a self-report questionnaire measured soccer competence (The Basic Needs Satisfaction in Sport Scale; BNSSS) as a control variable and parent-initiated motivational climate (Parent-Initiated Motivational Climate Questionnaire 2; PIMCQ-2) and peer motivational climate (Peer Motivational Climate in Youth Sport Questionnaire; PeerMCYSQ) as explanatory variables. At Time 2, the level of motivational autonomy (revised Sport Motivation Scale; SMS-II) was measured as an objective variable. After calculating correlations to confirm the relationships between the variables, a hierarchical multiple regression analysis was conducted, including an interaction term between parent and peer motivational climate. Results indicated that an anxiety-promoting parental motivational climate predicted low player autonomy under different peer motivational climates and did not support the hypothesis that a mastery-oriented peer motivational climate moderate negative parental influences. We discussed the importance of a mastery-oriented parental motivational climate in supporting player autonomy motivation during the developmental transition from childhood to adolescence.