This study investigated whether motivational climate was associated with prosocial and antisocial behavior in youth athletes directly and indirectly via social support (i.e., emotional and esteem support), perspective taking, and moral disengagement. A total of 275 youth team sport players (156 boys and 119 girls) aged 11 to 16 years completed questionnaires assessing the study variables. Structural equation modeling revealed that mastery climate was positively associated with prosocial teammate behavior both directly and indirectly via social support. Mastery climate was also indirectly associated with prosocial opponent behavior via social support and perspective taking. Mastery climate was negatively associated with antisocial behavior toward opponents and teammates indirectly via social support, perspective taking, and moral disengagement. Performance climate was positively associated with antisocial behavior toward teammates directly and indirectly via moral disengagement: It was also indirectly associated with antisocial behavior toward opponents via moral disengagement. Our findings extend understanding about the variables that might explain relationships between motivational climate and moral conduct in youth athletes. Implications for sport practitioners are considered, especially in relation to approaches that could help foster moral character in young people through sport. (PsycINFO Database Record (c) 2018 APA, all rights reserved)