<p>Existing findings regarding the relationship between rumination and mental health are conflicting, necessitating more systematic analysis to clarify the contradictions. This PRISMA-compliant study explores the relationship between rumination and positive indicators (i.e., well-being) and negative indicators (i.e., sleep quality) of mental health, as well as potential moderators. The literature search yielded 49 articles, including 180 effect sizes and 45287 participants. Meta-analysis revealed that rumination was significantly negatively correlated with well-being (<i>r</i> = -0.252, <i>p</i> &lt; 0.001), and positively correlated with sleep quality (<i>r</i> = 0.333, <i>p</i> &lt; 0.001). In the moderation analysis, the dimension of rumination (i.e., reflection, brooding, and symptom-focused rumination) exerted a significant moderating effect in the relationship between rumination and sleep quality. Likewise, the dimension of well-being (i.e., life satisfaction, positive emotions, negative emotions, and self-esteem) significantly moderated the association between rumination and well-being. Rumination appears to reduce sleep quality and well-being among adolescents, with research demonstrating a cross-cultural and cross-group consistency in its threat to adolescent mental health.</p>