Purpose <p>Engaging in recommended levels of physical activity (PA) provides numerous health benefits for children and adolescents. However, the relationship between knowledge of PA guidelines and behaviour change stages remains complex and not well understood. We analyzed cross-sectional baseline data from the Movimente study, a cluster-randomized controlled trial involving Brazilian adolescents (7th to 9th grades).</p> Methods <p>A standardized questionnaire measured knowledge of PA guidelines, stages of behaviour change, and sociodemographic factors. Multinomial logistic regression models were employed to examine the association between knowledge and behaviour change stages, adjusting for sex, age, and socioeconomic status.</p> Results <p>Among the 822 participants (mean age 13&#xa0;years, 52% female), only 22.4% correctly knew the PA guidelines, and only 33.5% were in the maintenance stage. Those who overestimated the guidelines were more likely to be in the maintenance stage of behaviour change compared to those who reported knowing the guidelines. Conversely, participants unaware of the guidelines were more likely to be in precontemplation and contemplation stages.</p> Conclusion <p>The study highlights a limited awareness of PA guidelines among adolescents and demonstrates a link between knowledge and behaviour change stages. Those overestimating guidelines tended to be more active, while those unaware faced barriers to initiation. The findings underscore the importance of disseminating accurate guidelines and creating interventions that address varying levels of knowledge, especially in underserved communities. These results contribute to a better understanding of the role of knowledge in behaviour change and provide insights for designing effective interventions to promote physical activity among adolescents.</p> Trial Registration <p>The study was pre-registered at Clinicaltrials.gov identifier NCT02944318.</p>