Objective To evaluate the impact of implementing a smoke-free policy in an Australian high-security forensic psychiatry in-patient hospital. Methods Focus groups (n≤21) and surveys with both patients (n≤45) and staff (n≤111; 53.2% nurses) elicited participants' experience of and attitudes towards an introduced smoke-free policy. A follow-up survey elicited the impact of the policy on 15 patients' smoking practice after discharge. Results Eighty-five per cent of patients stated it was easier to quit smoking when no one else smoked. Over half the discharged patients surveyed (58%) continued to not smoke after discharge, despite almost half the staff (41%) perceiving that patients were unlikely to quit long-term. Smoking staff were significantly more pessimistic than non-smoking staff. Many patients (69%) perceived that their health had improved as a result of not smoking. Most staff (80%) viewed nicotine dependence treatment as important, but fewer (66%) felt confident to support patients to stop smoking. Increased patient violence and management difficulties expected by staff were not realised. Conclusions A smoke-free policy can be successfully implemented in forensic psychiatry in-patient units. Nursing staff are a large and important group who need particular support to implement a smoke-free policy into practice effectively, particularly those who are smokers. Continuity of care as part of a coordinated policy and service response is needed. What is known about the topic? Healthcare settings are increasingly going totally smoke free, with mental health facilities the slowest to implement these policy changes. This is due, in part, to the fear of possible increases in violence and aggression among a patient population who have a high prevalence of smoking. What does this paper add? This paper adds to the evidence base that implementing a smoke-free policy in mental health in-patient hospitals is not only feasible, but also has substantial benefits for both staff and patients. In particular, it highlights the policy and practice implications that can assist mental health facilities to go smoke free. What are the implications for practitioners? A smoke-free policy can be successfully implemented in forensic psychiatry in-patient units; however, more support and education for staff is needed to increase their confidence and capacity to implement the policy consistently across service domains.