Abstract: Background/aims: Suicidal Ideation (SI) is a risk factor for suicide, a leading cause of death amongst young men globally. In this study we assess whether sustaining a serious physical combat injury is associated with SI and whether leaving service mediates this association. Methods: We analysed data from male UK Armed Forces personnel who sustained a combat injury in Afghanistan and a frequency-matched comparison group who did not sustain such an injury (the ADVANCE cohort). SI was measured from the Patient Health Questionnaire-9 item 'thoughts that you would be better off dead or of hurting yourself in some way'. Results: Approximately, 11.9% (n = 61) of the uninjured group, 15.3% (n = 83) of the overall injured group, 8.5% (n = 13) of an Amputation injury (AI) subgroup and 17.6% (n = 70) of a Non-Amputation Injury (NAI) subgroup reported SI in the past 2 weeks. The NAI subgroup reported greater likelihood of SI (Relative Risk Ratio (RR) = 1.44, 95% confidence interval (CI) [1.04, 2.00]) compared to the comparison group, whereas the overall injured group (RR = 1.23, 95% CI [0.90, 1.68]) and AI subgroup (RR = 0.65, 95% CI [0.36, 1.18]) did not. Leaving service fully mediated the association between sustaining a NAI and SI (natural direct effect RR = 1.08, 95% CI [0.69, 1.69]). Conclusions: UK military personnel with NAI reported significantly higher rates of SI compared to demographically similar uninjured personnel, while those who sustained AIs reported no significant difference. Leaving service was associated with greater rates of SI for both injured and uninjured personnel and fully mediated the association between sustaining a NAI and SI.