Parent advocacy is important for the transition outcomes of autistic youth. However, it is unclear whether parent advocacy efforts support or stifle youths' self-determination. This study examined concurrent (n = 180) and longitudinal (n = 134) associations between parent advocacy and transition-aged autistic youths' self-determination (as reported by parents) and explored whether individual and family characteristics moderated this relationship. Cross-sectional results indicated a positive association between parent advocacy and self-determination for youth with higher adaptive behavior, lower externalizing behavior, and higher parent-child relationship quality. Longitudinal results demonstrated that change in parent advocacy related to change in self-determination for youth with lower adaptive behavior and higher externalizing behavior. Findings suggest that targeting parent advocacy could enhance self-determination skills in autistic youth.