Rumination has been shown to be positively associated with suicide ideation. Yet, only few studies have attempted to explain potential mediators of this association. Perceptions of entrapment are a core component of recent psychological models of suicidality and might mediate the relationship between rumination and suicide ideation. Possible mediator effects were investigated in an online sample (n = 142) and a clinical sample (n = 226) of adults receiving outpatient psychotherapy. Results demonstrated that perceptions of entrapment fully mediated the association between ruminative thinking and suicide ideation. However, the reverse relationship, where the association between entrapment and suicide ideation is mediated by ruminative thinking, was not supported. These findings suggest that the relationship between rumination and suicide ideation is explained by perceptions of entrapment. Theoretical and clinical implications are discussed. Copyright © 2015 John Wiley & Sons, Ltd. Key Practitioner Message Ruminative thinking and perceptions of entrapment are associated with suicidal ideation and suicidal behavior. Perceptions of entrapment fully mediate the association between ruminative thinking and suicide ideation. Clinically, it could be useful to incorporate perceptions of entrapment into the psychosocial risk assessment of persons contemplating suicide.