BACKGROUND: Japanese forensic mental health services for patients with psychiatric disorders under the Medical Treatment and Supervision Act was initiated in 2005; however, the prognosis of those patients is not well-known, particularly regarding mortality and suicide. This study aimed to evaluate the all-cause mortality and suicide rate in forensic psychiatric outpatients who had been discharged from forensic psychiatric wards in Japan. METHODS: Participants included 966 patients who had been discharged from forensic psychiatric wards. Data were collected from July 15, 2005 to July 15, 2018 at 29 of the 33 forensic psychiatric wards in Japan. Only the patients who provided written informed consent were included. We and collaborators at each forensic psychiatric ward identified demographic data of participants from the medical records for the inpatient treatment period. The reintegration coordinators, who belonged to the Ministry of Justice, investigated the prognosis of the participants during the outpatient treatment order period. We then connected demographic data and participants' prognosis for analysis. The crude rates (CRs) and standardized mortality ratios (SMRs) were calculated to analyze all-cause mortality and suicide rates. Univariate analysis was performed to examine the factors associated with all-cause mortality and suicide rates using the Cox proportional hazards ratio model. RESULTS: The participants included 3.3 times as many men (n=739) compared to women (n=227), and their combined mean age was 47.3 (SD=12.9). The most common primary psychiatric diagnosis was psychotic disorders (81.3%). The mean follow-up period was 790.2 days (SD=369.6). The total observation period was 2091.2 person-years. The CR for all-cause death was 812.9 per 100,000 person-years (95% CI [426.5, 1199.4]), while the SMR for all-cause death was 2.2 (95% CI [1.3, 3.5]). The CR for completed suicide was 478.2 per 100,000 person-years (95% CI [181.8, 774.6]). The suicide SMR was 17.9 (95% CI [8.6, 32.9]) overall, 7.7 (95% CI [2.5, 18.0]) for men, and 79.4 (95% CI [25.8, 185.2]) for women. Univariate analysis showed that women had higher completed suicide risk than men (hazard ratio=3.599, 95% CI [1.041, 12.445]). CONCLUSION: The all-cause mortality and completed suicide rates were higher in participants than observed in the general population consistent with the results of previous international studies.