Smoking rates among people with serious mental illness are 3 to 4 times higher than the general population, yet currently there are no smoking cessation apps specifically designed to address this need. We report the results of a User Experience (UX) evaluation of a National Cancer Institute smoking cessation app, QuitPal, and provide user centered design data that can be used to tailor smoking cessation apps for this population.Two hundred forty hours of field experience with QuitPal, 10 hours of recorded interviews and task performances, usage logs and a self-reported usability scale, informed the results of our study. Participants were five individuals recruited from a community mental health clinic with a reported serious mental illness history. Performance, self-reports, usage logs and interview data were triangulated to identify critical usability errors and UX themes emerging from this population.Data suggests QuitPal has below average levels of usability, elevated time on task performances and required considerable amounts of guidance. UX themes provided critical information to tailor smoking cessation apps for this population, such as the importance of breaking down "cessation" into smaller steps and use of a reward system.This is the first study to examine the UX of a smoking cessation app among people with serious mental illness. Data from this study will inform future research efforts to expand the effectiveness and reach of smoking cessation apps for this highly nicotine dependent yet under-served population.Data from this study will inform future research efforts to expand the effectiveness and reach of smoking cessation apps for people with serious mental illness, a highly nicotine dependent yet under-served population.