People diagnosed with a mental illness are at higher risk of developing preventable chronic diseases; thus, health literacy improvements may have great potential to impact health outcomes for this typically underserved population. However, there is a dearth of research on health literacy of persons with severe mental illness. The purpose of this research was to investigate aspects of health literacy and identify factors associated with low literacy among adults with severe mental illness using three literacy assessment tools. Seventy-one adults with serious mental illness were assessed and a high proportion had limited literacy levels: 42 % with the Single Item Literacy Screener, 50 % with the Rapid Estimate of Adult Literacy in Medicine-Short Form, and 67 % with the Newest Vital Sign. Findings suggest that individuals with certain mental illnesses and lower functioning may have more difficulty understanding health information and have limited numerical literacy.