The purpose of this study was to examine the link between physical activity (PA) and mental health (MH) in children in terms of gender. A total of 750 subjects (52.9% boys, Mage = 10.88; SD = 0.75) reported the level of PA they were performing by means of a self-report questionnaire, and they also completed the Spanish version of the Strengths and Difficulties Questionnaire (SDQ). Results revealed statistically significant differences in regard to gender in the physical activity index (PAI),

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... externalizing of problems, and prosocial behavior. Boys presented high scores on the PAI and externalizing problems. Girls obtained higher values on the prosocial scale. In the prediction analysis, the PAI revealed a statistically significant relationship in linear models for externalizing and internalizing problems, as well as in both the linear and quadratic models in the prosocial scale for the case of male gender. For girls, the PAI revealed a statistically significant relationship in the quadratic model for internalizing problems and in both the linear and quadratic models for the prosocial scale. These findings are discussed in terms of previous research literature on PA and MH in children. Some important educational implications for teaching and learning physical education are suggested.