Previous research has indicated that community-based mental health services in Iran are restricted, leading to overcrowding in psychiatric wards. This overcrowding has been linked to a range of problems, such as violence, suicide and medical errors. Despite the abundance of research on patient safety, there is still a lack of understanding regarding how mental health nurses (MHNs) create a secure environment within these wards. This study focused on exploring a safe environment provided by MHNs in inpatient psychiatric wards at Farshchian (Sina) Hospital, Hamadan, Iran. An explanatory mixed-methods study was conducted. Initially, the Safe Environment Scale was distributed to all MHNs (n = 48) working in three wards at Farshchian (Sina) Hospital to evaluate the current status. The scale measured two dimensions, and descriptive statistics were used to analyse the collected data. Subsequently, 20 MHNs were selected for semi-structured interviews using purposeful sampling at the same hospital to interpret and fill gaps in the quantitative findings. The data collected from the interviews were analysed using conventional content analysis. The perception and engagement of MHNs in creating a safe environment in the inpatient psychiatric wards were found to be at a medium level, according to the Safe Environment Scale (mean ± SD, 14.67 ± 4.18 and 85.27 ± 17.57, respectively). The qualitative study identified several categories in the results, including 'Hyper-vigilance to safety and security environment', 'Therapeutic communication gap', 'Nurse burnout', 'Staff safety and security need' and 'Environmental safety hazards'. MHNs employ a hyper-vigilant strategy to guarantee a secure atmosphere within psychiatric wards. However, this approach may inadvertently impede the establishment of a safe environment and even diminish MHNs' perception and involvement in its maintenance. According to our research, it appears that MHNs need to improve their education and training in order to successfully implement the vigilance strategy for establishing a secure environment. Additionally, it is essential for them to prioritize therapeutic communication with patients, as this plays a vital role in promoting a safe environment within inpatient psychiatric wards.