Introduction: Co-rumination, characterized by excessively discussing problems and dwelling on negative affect within a dyadic friendship, has been associated with adolescents' symptoms of depression, anxiety and perceived stress-collectively referred to as psychological distress. This study explored whether co-rumination moderates the association between perceived best friend support and psychological distress. Methods: The study included 187 adolescents (52.9% girls; 88.0% Dutch ethnic background) recruited from two cohorts between March 2017 and July 2019. Assessments took place at two time points: symptoms of depression, anxiety, and perceived stress were assessed via self-report measures at the final grade of primary school (T1; Mage = 11.8 years) and in secondary school (T2; Mage = 13.3 years). Co-rumination and perceived best friend support were measured via self-report in secondary school. Results: Findings indicate that best friend support was associated with lower psychological distress and conversely, co-rumination was associated with higher psychological distress while adjusting for prior distress symptoms. Moderation analysis revealed that moderate levels of co-rumination (relative to the samples mean) decreased the positive effects of perceived best friend support on symptoms of depression (B = 0.06, SE = 0.03, 95% CI [0.00, 0.11], p = 0.05, [beta] = 0.11) and perceived stress (B = 0.06, SE = 0.01, 95% CI [0.03, 0.08], p = 0.000, [beta] = 0.10). At very high levels of co-rumination (relative to the samples mean), best friend support exacerbates perceived stress. Discussion: This study underscores the potential negative impact of co-rumination in supportive peer relationships and recommends promoting awareness of the risk of co-rumination while building a repertoire of (dyadic)emotion regulation strategies.