Grief symptoms and a sense of injustice may be interrelated responses amongst persons exposed to mass conflict and both reactions may contribute to post-traumatic stress disorder (PTSD) symptoms. As yet, however, there is a dearth of data examining these relationships. Our study examined the contributions of grief and a sense of injustice to a model of PTSD symptoms that included the established determinants of trauma events, ongoing adversity and severe psychological distress. The study involved a large population sample (n = 2964, response rate: 82.4%) surveyed in post-conflict Timor-Leste. The survey sites included an urban administrative area (suco) in Dili, the capital of Timor-Leste and a rural village located an hour's drive away. Culturally adapted measures were applied to assess conflict related traumatic events (TEs), ongoing adversity, persisting preoccupations with injustice, symptoms of grief, psychological distress (including depressive symptoms) and PTSD symptoms. We tested a series of structural equation models, the final comprehensive model, which included indices of grief symptoms and injustice, producing a good fit. Locating grief symptoms as the endpoint of the model produced a non-converging model. In the final model, strong associations were evident between grief and injustice (β = 0.34, s.e. = 0.02, p < 0.01) and grief and PTSD symptoms (β = 0.14, s.e. = 0.02, p < 0.01). The sense of injustice exerted a considerable effect on PTSD symptoms (β = 0.13, s.e. = 0.03, p < 0.01). In addition, multiple indirect paths were evident, most involving grief and a sense of injustice, attesting to the complex inter-relationship of these factors in contributing to PTSD symptoms. Our findings support an expanded model of PTSD symptoms relevant to post-conflict populations, in which grief symptoms and a sense of injustice play pivotal roles. The model supports the importance of a focus on loss, grief and a sense of injustice in conducting trauma-focused psychotherapies for PTSD amongst populations exposed to mass conflict and violence. Further research is needed to identify the precise mechanisms whereby grief symptoms and the sense of injustice impact on PTSD symptoms.