Background <p>To clarify whether positive perfectionism is associated with exercise persistence in adolescents and to test a moderated serial mediation model.</p> Methods <p>A cross-sectional school-based survey was conducted among 1,032 students (12–18 years) in Dalian, China. Participants reported positive perfectionism, self-efficacy, exercise-related autonomous motivation, perceived social support, and exercise persistence. Regression with bootstrapping tested mediation and moderated mediation.</p> Results <p>Positive perfectionism was positively associated with exercise persistence. This association was statistically mediated by self-efficacy and autonomous motivation in sequence. Perceived social support moderated the autonomous motivation–exercise persistence link, such that the serial indirect association was stronger at higher levels of perceived social support.</p> Conclusions <p>Positive perfectionism was associated with greater exercise persistence via higher self-efficacy and autonomous motivation, and this indirect association was stronger when perceived social support was higher. Longitudinal or experimental studies are needed to clarify causal directions.</p>