Objective <p>Prior studies have found associations between sleep disturbances and suicide ideation among adolescents. However, there is limited prospective research examining cognitive mechanisms that explain this relationship. This present longitudinal study aimed to examine the mediating role of rumination in the relationship between sleep quality and suicide ideation over time. We hypothesized that low sleep quality would be associated with increased suicide ideation severity over time and that rumination would mediate the relationship.</p> Methods <p>Adolescents (<i>N</i> = 117), ages 12–19, who presented to hospitals or outpatient clinics with suicide ideation or attempt completed self-report measures of sleep quality and suicide ideation severity at baseline, rumination at 3-month follow-up, and depressive symptoms and suicide ideation at 12-month follow-up.</p> Results <p>There was no statistically significant direct relation between sleep quality and suicide ideation. However, rumination and depressive symptoms serially mediated the relationship between sleep quality and suicide ideation, suggesting an indirect relationship. Interestingly, rumination did not predict later sleep quality.</p> Conclusions <p>Improving sleep quality may reduce rumination, which, in turn, may reduce depressive symptoms and lower SI severity. Improving sleep quality may ameliorate the impact of cognitive mechanisms—like rumination—that may indirectly exacerbate suicide ideation severity through depressive symptoms.</p>