Research has demonstrated the predictive effect of peer victimization on adolescent suicidal ideation. However, few studies have explored the bidirectional relations between peer victimization and suicidal ideation and how insomnia symptoms mediate these bidirectional relations. The present study examined reciprocal relations between peer victimization (i.e., physical and relational victimization) and suicidal ideation and the potential mediating role of insomnia symptoms and sex differences by disentangling between- and within-person effects. A total of 4731 students (44.9% girls; Mage = 10.91 years, SD = 0.52) participated in a four-wave longitudinal study with 6-month intervals. Results from random intercept cross-lagged panel modeling indicated significant positive associations between peer victimization, insomnia symptoms, and suicidal ideation at the between-person level. At the within-person level, significant sex differences were identified in the bidirectional associations between physical or relational victimization, insomnia symptoms, and suicidal ideation. Specifically, boys showed significant bidirectional associations between physical victimization and suicidal ideation. In contrast, for girls, suicidal ideation significantly predicted subsequent physical victimization, but the reverse pathway was not significant. Furthermore, no significant cross-lagged associations were found between relational victimization and suicidal ideation among boys. However, for girls, relational victimization and suicidal ideation demonstrated significant cross-lagged effects. Significant bidirectional associations between physical victimization and insomnia symptoms were observed in both boys and girls. For girls, insomnia symptoms significantly mediated the bidirectional relations between physical victimization and suicidal ideation, but the bidirectional relations between relational victimization and insomnia symptoms were not supported. For boys, while both physical and relational victimization significantly predicted insomnia symptoms, insomnia symptoms did not predict suicidal ideation. In the reverse pathway, insomnia symptoms mediated the pathway from suicidal ideation to physical victimization among boys. The findings underscore the importance of considering both sex differences and insomnia symptoms in understanding the pathways linking peer victimization to suicidal ideation, offering valuable insights for targeted interventions.