The Environmental Stress Hypothesis (ESH) is a causal theoretical framework that provides a flexible context for understanding factors that mediate the relationship between low motor proficiency and internalizing problems in children. The purpose of the present study was to use the ESH framework to determine whether body mass index (BMI), physical activity levels, psychosocial health and physical health, self-efficacy, perceived social status, prosocial behavior and externalizing problems were potential mediators between motor proficiency and internalizing factors in a population of Brazilian children. 431 children aged 7- to 10 years (240 females, 191 males) participated in the study. The variables were measured with the Movement Assessment Battery for Children, 2nd ed. (MABC-2), the Pediatric Quality of Life Inventory (PedsQL), the Strengths and Difficulties Questionnaire (SDQ), the MacArthur Subjective Social Status Scale (MacArthur SSS), the Self-efficacy Sense Assessment Roadmap (RASAE), and the Physical Activity Questionnaire (PAQ). The results indicated a direct relationship between motor proficiency and internalizing problems in a population of Brazilian children, with externalizing problems being the only variable mediating that relationship. To the best of our knowledge, this is the first time that externalizing problems were tested in the context of the ESH. Understanding and evaluating potential mediators in the relationship between motor proficiency and internalizing problems using the ESH framework is essential to promote prevention policies and interventions for school-age children. •The relationship between motor proficiency and internalizing problems is mediated by externalizing problems.•Physical activity did not mediate the relationship between low motor proficiency and internalizing problems.•This study extends the ESH by adding a mediating pathway that had not previously been addressed.