Although previous work has consistently identified positive associations between co-rumination and rumination during adolescence, little to no research has examined how this relationship operates on the person-specific level. The current study aimed to extend current developmental theories of co-rumination and rumination by examining within-person associations between these constructs. Survey data was collected from 1502 adolescents (Mage = 13.20; 52% girls; 52% non-Hispanic White) every six-months across the span of 3.5 years. The results showed that at time-points when adolescents reported co-ruminating more than their usual level, they reported concurrent increases in rumination. This association was stronger for boys and strengthened over time. Despite substantial between-person heterogeneity, 97% of adolescents showed positive associations between co-rumination and rumination. This research has important implications for mental health professionals, school systems, and parents who may look to teach adolescents about effective emotion-regulation.