Little data exist to guide suicide assessment protocols in preadolescent youth. Using the Adolescent Brain Cognitive Development (ABCD) data (N = 10,010) at baseline (ages 9–10) and 24-month follow-up (ages 11–12), this report investigates informant agreement/disagreement in caregiver- and youth- reports of suicidal ideation and their associations with youth sex assigned at birth and symptomatology across preadolescence. Using the Kiddie Schedule for Affective Disorders and Schizophrenia (K-SADS) at both timepoints, four informant discrepancy groups were created from caregiver- and youth- reported suicidal ideation: (1) Concordant No; (2) Concordant Yes; (3) Discordant Caregiver Yes, Youth No; (4) Discordant Caregiver No, Youth Yes. Internalizing and externalizing symptoms were measured at the 24-month follow-up using the caregiver-report Child Behavior Checklist and the youth-report ABCD Brief Problem Monitor. Results indicated low-to-fair caregiver-youth agreement for youth suicidal ideation across preadolescence. Suicidal ideation reported by youth, but not caregivers, demonstrated a shift in prevalence at the follow-up, with females surpassing males in self-reported endorsements. Finally, informant groups at both time points were associated with caregiver- and youth-reported youth symptomatology at the 24-month follow-up, and associations varied by reporter. Findings demonstrate the importance of leveraging both caregiver and youth reports to assess preadolescent suicidal ideation and highlight the clinical utility of informant discrepancies in assessing suicide risk. The accurate assessment of suicidal ideation in preadolescents is essential to curb the increasing rates of preadolescent suicide and identify at-risk youth for interventions prior to even larger uptick of suicide in adolescence.