Individuals diagnosed with serious mental illness (SMI) have greater trauma exposure and are at increased risk for posttraumatic stress disorder (PTSD). However, PTSD is rarely documented in their clinical records. This study investigated the predictors of PTSD documentation among 776 clients with SMI receiving public mental health services, who had probable PTSD as indicated by a PTSD Checklist score of at least 45. Only 5.3% of clients had PTSD listed as a primary diagnosis, and 8.4% had PTSD as a secondary diagnosis, with a total 13.7% documentation rate. PTSD documentation rate was highest for clients with major depression (18.8%) compared to those with schizophrenia (4.1%) or bipolar disorder (6.3%). Factors that predicted a lower likelihood of having a chart diagnosis of PTSD included being diagnosed with schizophrenia/schizoaffective disorder or bipolar disorder. Factors that predicted a higher likelihood of having a chart diagnosis of PTSD included being of non-white race, being female, and experiencing eight or more types of traumatic events. Findings highlight the need for PTSD screening and trauma informed care for clients with SMI receiving public mental health services.