Post-traumatic stress disorder (PTSD) is a common mental health disorder that can affect those who have experienced a traumatic event. Despite the availability of different treatment options for PTSD, there are several barriers that prevent some patients from receiving treatment. To overcome these barriers, mobile health (mHealth) apps have been developed to allow access to therapeutic and self-assessment tools outside the clinic. Our review of literature shows that the three mostly used apps (PTSD Coach, PE Coach, and CPT Coach) are not empirically evaluated and very little information is available for the process used in design and development of these tools. This paper documents a usability study of the most popular PTSD mHealth app; PTSD Coach. Findings indicate that the learning component of the app provides useful information, the assessment is effective in keeping track of the symptoms, and that some of the tools provided can help mitigate some of the symptoms. However, the color scheme, lack of personalization options, and lack of clarity on the mitigation techniques was deemed to affect usability.