This study was designed to investigate long-term clinical outcomes of risperidone long-acting injectable (RLAI) in patients with schizophrenia or schizoaffective disorder. An open-label, 48-week, prospective study of RLAI treatment was carried out at 63 centers in South Korea. Initial and maintenance dosage of RLAI were adjusted according to clinical judgment. Efficacy was measured by the remission rate, continuation rate, and changes in the clinical measurements such as eight items of the Positive and Negative Symptom Scale (PANSS), the Clinical Global Impression – Severity, and the Schizophrenia Quality of Life Scale. In terms of the safety, Simpson-Angus rating Scale, adverse events (AEs), and BMI were investigated. Of the 522 patients who were enrolled, 472 patients who had been assessed on the eight items of PANSS at baseline and at least once during RLAI treatment were included in the intention-to-treat (ITT) population. The per-protocol (PP) population included 184 patients (39.0%), who completed all assessments during 48 weeks of the follow-up period. Total scores of eight items of PANSS, Clinical Global Impression – Severity, and Schizophrenia Quality of Life Scale were reduced significantly from baseline to endpoint in both ITT and PP populations. The mean dose (SD) of RLAI was 33.2 (7.6) mg. In the PP population, the number of patients who scored 1–3 on eight items of PANSS were 47 (25.5%) at baseline and 144 (78.3%) at 48 weeks. According to the remission defining as scores 1–3 on eight items of PANSS sustaining of at least 6 months’ duration by Andreasen, the numbers of patients who achieved remission were 45 (24.5%) at 24 weeks and 120 (65.2%) at 48 weeks. A significant decrease in the mean score of Simpson-Angus rating Scale and a significant increase in BMI over time in last observation carried forward were observed, and patients who fulfilled the remission criteria during the study showed more weight gain than those who did not. During the study period, a total of 645 AEs were noted in 233 patients (49.3%) who were included in the ITT population. Sixty-nine serious AEs in 51 patients were reported, but all of them were not directly attributable to administration of RLAI. This prospective, open-label study showed improvements in symptom and AEs and a significant increase in BMI during 48 weeks of biweekly RLAI treatment. The rate of study completion was 39.0% and the remission rate among those who completed the study was 65.2%. None of the serious AEs were directly related to the administration of RLAI.