Subthreshold post-traumatic stress disorder (PTSD) is more prevalent than PTSD, yet its role as a potential risk factor for PTSD is unknown. To address this gap, we analysed data from a 7-year, prospective national cohort of USA veterans. Of veterans with subthreshold PTSD at wave 1, 34.3% developed PTSD compared with 7.6% of trauma-exposed veterans without subthreshold PTSD (relative risk ratio 6.4). Among veterans with subthreshold PTSD, specific PTSD symptoms, greater age, cognitive difficulties, lower dispositional optimism and new-onset traumas predicted incident PTSD. Results suggest that preventive interventions targeting subthreshold PTSD and associated factors may help mitigate risk for PTSD in USA veterans.