The aim of this study was to examine whether depression, hopelessness and perceptions of defeat and entrapment mediated the effects of posttraumatic stress disorder (PTSD) symptoms on suicidal behavior. Participants were 73 individuals (mean age=29.2, S.D.=10.9, 79.5% female) diagnosed with current or lifetime PTSD who reported at least one PTSD symptom in the past month. Participants completed a series of self-report measures assessing depression, hopelessness and perceptions of defeat and entrapment. The Clinician Administrated Posttraumatic Scale for DSM-IV was administered to assess the presence and severity of PTSD symptoms. The results of Structural Equation Modeling supported a model whereby perceptions of defeat and entrapment fully mediated the effects of PTSD symptom severity upon suicidal behavior. The finding that perceptions of defeat and entrapment mediate the relationship between PTSD symptom severity and suicidal behavior was replicated in a subgroup of participants (n=50) who met the full criteria for a current PTSD diagnosis. The results support a recent theoretical model of suicide (The Schematic Appraisal Model of Suicide) which argues that perceptions of defeat and entrapment have a key role in the development of suicidal behaviors. We discuss the clinical implications of the findings.