The purpose of this study was to examine the relationship between sleep problems and suicidal ideation in depressive patients. Evidence from diverse settings and populations indicates that sleep problems are associated with suicidal ideation. However, relationships between them are not well defined. This study was conducted in order to explore whether poor sleep was associated with suicidal ideation above and beyond depression and whether specific domains of sleep were related to suicidal ideation. We also determined whether the association between sleep problems and suicidal ideation was mediated by depression. Patients aged 18-65 years from an outpatient clinic at Seoul St. Mary's Hospital were recruited for this study. From September 2010 to November 2017, a total of 909 participants were included in the final sample. The Beck Depression Inventory (BDI) and Pittsburgh Sleep Quality Index (PSQI) were used to assess psychiatric symptoms. Independent samples t-test, chi-square test, Pearson correlation analyses, hierarchical multiple regression analyses, and mediation analyses were performed using SPSS PROCESS macro. The majority of participants with suicidal ideation also had sleep problems (94.9%). After controlling for age, marital status, and depressive symptoms, total sleep problems estimated by the PSQI global score were also significant associated with suicidal ideation. Among seven sleep components derived from the PSQI, several components including cough or snore loudly, have bad dreams, and use sleep medication were associated with increased suicide risk. Also, the relationship between sleep problems and suicidal ideation was mediated by depressive symptoms indirectly. There was no convincing direct relationship between sleep problems and suicidal ideation. Investigating the pathways which connect sleep problems and suicidality is fundamental to the development of suicide prevention. While it might be premature to suggest specific interventions, it would be important for clinicians to consider evaluating and managing sleep problems in the context of suicidality. Copyright © 2019. Published by Elsevier B.V.