PTSD is common among Veterans with serious mental illness (SMI). Co-occurring PTSD and SMI lead to poorer mental health and physical functioning than either diagnosis alone. Despite known high prevalence rates of PTSD in SMI populations as well as disparities in prevalence and treatment use for Black, Indigenous, and other people of color (BIPOC), little research has been done to: a) evaluate leading treatments for PTSD in individuals with SMI, and b) develop culturally responsive methods to integrate with PTSD treatments for SMI Veterans. This study aims to address research and clinical gaps by: a) testing the feasibility and acceptability of Written Exposure Therapy (WET), a VA evidence-based psychotherapy for PTSD in Veterans with SMI, and b) incorporating culturally responsive assessment methods. Results from this study will inform whether WET and culturally responsive assessment are feasible to implement, acceptable to Veterans with SMI, and worth examining in standard or optimized form in a larger clinical trial.