BACKGROUND: While adherence to the WHO 24-hour movement guidelines has been linked to improved health outcomes, its relationship with brain morphology remains underexplored, particularly in understudied populations such as Latin American adolescents. METHODS: This cross-sectional study examined brain morphology in 58 Chilean adolescents using structural T1-weighted magnetic resonance imaging. Brain volumes, including total gray matter, cortical, subcortical, hippocampal, and amygdala regions, were analyzed. Adherence to moderate-to-vigorous physical activity (MVPA) and sleep guidelines was assessed via accelerometry, while screen time was self-reported. Linear regression models were used to explore associations between guideline adherence and brain volumes, with p-values corrected using the FDR method. RESULTS: Independently, most adolescents met only the screen time guideline (55 %), while fewer met sleep duration (12 %) or MVPA based on the sample-specific cutoff (27 %), with none meeting the international MVPA criterion. In combination, only 9 % adhered to both sleep and screen time recommendations, and no participants met all three guidelines. Notably, only adherence to screen time recommendations showed a trend-level association with lower total gray matter, cortical, subcortical, hippocampal, and amygdala volumes (FDR p-value <
0.100). CONCLUSION: This study suggests that adolescents' daily habits-screen time, physical activity, and sleep-have complex relationships with brain development, which may sometimes diverge from expected patterns, as seen in the case of meeting screen time guidelines. As the first study conducted in Latin American adolescents, these suggests proposes that the relationship between these behaviors and brain development may differ from what has been reported in high-income countries, highlighting the need for further research in diverse socioeconomic and cultural contexts.