Abstract: Objective: Trauma-exposed veterans may be more likely to experience posttraumatic stress disorder (PTSD), chronic pain, and sleep disturbance together rather than in isolation. Although these conditions are independently associated with distress and impairment, how they relate to social functioning and suicidal ideation (SI) when experienced comorbidly is not clear. Method: Using longitudinal data on 5,461 trauma-exposed U.S. veterans from The Veterans Metrics Initiative study and self-reported disorders, we assessed (a) the extent to which PTSD co-occurs with sleep disturbance and chronic pain (CP); (b) the relationship of PTSD in conjunction with sleep disturbance and chronic pain with later social functioning and SI; and (c) the extent to which social functioning mediates the impact of multimorbidity on SI. Results: At approximately 15 months postseparation, 90.5% of veterans with probable PTSD also reported sleep disturbance and/or CP. Relative to veterans without probable PTSD, veterans with all 3 conditions (n = 907) experienced the poorest social functioning (B = -.56, p < .001) and had greater risk for SI (OR = 3.78, p < .001); Social functioning partially mediated the relationship between multimorbidity and SI. However, relative to those with PTSD alone, sleep disturbance and CP did not confer greater risk for SI. Conclusions: Although these Findings underscore the impact of PTSD on functioning and SI, they also highlight the complexity of multimorbidity and the importance of bolstering social functioning for veterans. (PsycInfo Database Record (c) 2023 APA, all rights reserved).