Studies examining Posttraumatic Stress Disorder symptom clusters, suicidal ideation, and variables described in the Joiner's interpersonal theory of suicidal behavior were cross-sectional. We tested whether the relation between the Posttraumatic Stress Disorder symptom clusters and suicidal ideation is mediated by perceived burdensomeness and thwarted belongingness (variables of the interpersonal theory of suicidal behavior) in military personnel with current suicidal ideation using longitudinal design. DSM-IV model with reexperiencing, avoidance, numbing, and hyperarousal clusters was used. Structural equation modelling was used to test separate models for each symptom cluster with a symptom cluster at baseline, month 1 perceived burdensomeness and thwarted belongingness, and month 3 suicidal ideation, controlling for baseline values of the month 1 and month 3 variables. Analysis of direct non-mediation models showed that baseline reexperiencing (p = .08) and avoidance (p = .07) symptom clusters marginally predicted month 3 suicidal ideation. The mediation analyses showed indirect effects from baseline reexperiencing, avoidance, and hyperarousal to suicidal ideation at month 3 through perceived burdensomeness at month 1. Thwarted belongingness did not mediate the relations between symptom clusters and suicidal ideation. The mechanism driving development of suicidal ideation in military personnel with PTSD may be different from the mechanism in other samples. Doing a phone interview limited us to use questionnaires instead of a clinical interview. Our findings suggest that interventions targeting perceived burdensomeness in military personnel with reexperiencing, avoidance, and hyperarousal symptoms may be beneficial to reduce suicidal ideation. Copyright © 2020. Published by Elsevier B.V.