Objective: Posttraumatic stress disorder (PTSD) is highly prevalent in the population, but relatively few affected individuals receive treatment for it. Smartphone applications (apps) could help address this unmet need by offering sound psychoeducational information and evidence-based cognitive behavioral coping tools. We conducted a randomized controlled trial to assess the efficacy of a free, publicly available smartphone app (PTSD Coach) for self-management of PTSD symptoms. Method: One hundred 20 participants who were an average of 39 years old, mostly women (69.2%) and White (66.7%), recruited primarily through online advertisements, were randomized to either a PTSD Coach (n = 62) or a waitlist condition (n = 58) for 3 months. Web-administered self-report measures of PTSD, PTSD symptom coping self-efficacy, depression, and psychosocial functioning were conducted at baseline, posttreatment, and 3 months following treatment. Results: Following the intent-to-treat principle, repeated-measures analyses of variance (ANOVAs) revealed that at posttreatment, PTSD Coach participants had significantly greater improvements in PTSD symptoms (p = .035), depression symptoms (p = .005), and psychosocial functioning (p = .007) than did waitlist participants; however, at posttreatment, there were no significant mean differences in outcomes between conditions. A greater proportion of PTSD Coach participants achieved clinically significant PTSD symptom improvement (p = .018) than waitlist participants. Conclusion: PTSD Coach use resulted in significantly greater improvements in PTSD symptoms and other outcomes relative to a waitlist condition. Given the ubiquity of smartphones, PTSD Coach may provide a wide-reaching, convenient public health intervention for individuals with PTSD symptoms who are not receiving care.