Children who are victimized by their peers are at increased risk of developing emotional difficulties and disorders (e.g., mental health diagnoses); conversely, children experiencing emotional difficulties may also be more likely to experience peer victimization. Sleep quality could serve as a protective buffer or risk factor in these longitudinal associations. In the present study, we examined bidirectional pathways between peer victimization and emotional difficulties, testing sleep quality as a potential moderator. Children (N = 293 ages 8-11; 52% girls) in grades 3-5 completed measures of emotional difficulties (anxiety, depression, irritability, emotion coping, and emotion dysregulation), peer victimization, and sleep quality in the fall (T1) and spring (T2) semesters of one school year. Path models, controlling for covariates and stabilities, showed that peer victimization at T1 predicted higher levels of anxiety, depression, irritability, and poor emotion coping at T2. Sleep quality moderated one path: the link between T1 peer victimization and T2 emotion dysregulation, which was positive and significant only among those with high sleep quality; those with low sleep quality at T1 showed moderate levels of dysregulation irrespective of victimization. Longitudinal paths from T1 emotional difficulties to T2 victimization were all nonsignificant. Model results were not moderated by grade or gender. Overall, results support the unidirectional conclusion that peer victimization contributes to various forms of emotional difficulties, but not vice versa. Sleep quality is relevant as a moderator, underscoring the need for further research. Findings suggest implications for prevention and intervention efforts to promote social-emotional development in school settings.