The current study examined the transdiagnostic nature of rumination in the development of childhood depression and aggression by examining the relation between two forms of rumination, sadness and anger, in a single study and assessing their unique and shared behavioral correlates. A community sample of 254 children (ages 7-14, 50.4% female, 66.5% Caucasian) completed self-report measures of rumination and depressive symptoms, and peer nominations of aggressive behaviors. Hierarchical regression analyses were conducted to assess unique and shared behavioral correlates. Anger rumination uniquely predicted aggression (β = .40, p < .001) and depressive symptoms (β = .62, p < .001), controlling for sadness rumination. Sadness rumination, controlling for anger rumination, did not predict depressive symptoms (β = -.10, p = .10) and negatively predicted aggressive symptoms (β = -.21, p = .003). In addition, a significant interaction between sadness rumination and anger rumination on aggressive behaviors was observed (β = -.24, p < .001), such that children who reported high anger and low sadness rumination tendencies were perceived as more aggressive by their peers than other children, including those with high levels of anger and sadness rumination. These results offer support for anger rumination as a transdiagnostic factor for children's depressive symptoms and aggression. Sadness rumination did not uniquely predict depressive symptoms, although it did moderate the association between anger rumination and aggression. These findings underscore the importance of assessing both anger and sadness rumination for increasing our understanding of children's risk for depression and aggression.