We examined the efficacy of a brief, accessible, nonstigmatizing online intervention-writing expressively about transitioning to civilian life. U.S. Afghanistan and Iraq war veterans with self-reported reintegration difficulty (N = 1,292, 39.3% female, M = 36.87, SD = 9.78 years) were randomly assigned to expressive writing (n = 508), factual control writing (n = 507), or no writing (n = 277). Using intention to treat, generalized linear mixed models demonstrated that 6-months postintervention, veterans who wrote expressively experienced greater reductions in physical complaints, anger, and distress compared with veterans who wrote factually (ds = 0.13 to 0.20; ps <.05) and greater reductions in PTSD symptoms, distress, anger, physical complaints, and reintegration difficulty compared with veterans who did not write at all (ds = 0.22 to 0.35; ps ≤.001). Veterans who wrote expressively also experienced greater improvement in social support compared to those who did not write (d = 0.17). Relative to both control conditions, expressive writing did not lead to improved life satisfaction. Secondary analyses also found beneficial effects of expressive writing on clinically significant distress, PTSD screening, and employment status. Online expressive writing holds promise for improving health and functioning among veterans experiencing reintegration difficulty, albeit with small effect sizes.