Objective: The aim of this study was to understand the knowledge, beliefs, barriers, and behaviors of mental health professionals about physical activity and exercise for people with mental illness. Methods: The Portuguese version of The Exercise in Mental Illness Questionnaire was used to assess knowledge, beliefs, barriers, and behaviors about exercise prescription for people with mental illness in a sample of 73 mental health professionals (68.5% women, mean age = 37.0 years) from 10 Psychosocial Care Units (Centros de Atenção Psicossocial) in Porto Alegre and Canoas, state of Rio Grande do Sul, Brazil. Results: Most of respondents had received no formal training in exercise prescription. Exercise ranked fifth as the most important treatment, and most of the sample never or occasionally prescribed exercise. The most frequently reported barriers were lack of training in physical activity and exercise prescription and social stigma related to mental illness. Professionals who themselves met recommended physical activity levels found fewer barriers to prescribing physical activity and did so with greater frequency. Conclusion: Exercise is underrated and underused as a treatment. It is necessary to include physical activity and exercise training in mental health curricula. Physically active professionals are more likely to prescribe exercise and are less likely to encounter barriers to doing so. Interventions to increase physical activity levels among mental health professionals are necessary to decrease barriers to and increase the prescription of physical activity and exercise for mental health patients.