The aim of this study was to characterize the pharmacokinetics and to evaluate the safety of risperidone ISM in patients with schizophrenia or schizoaffective disorder after a single gluteal intramuscular injection at three different dose strengths (50, 75, and 100 mg). A total of 36 patients were randomized and blood samples were collected to measure the plasma concentrations. The pharmacokinetic of the active moiety was biphasic for all three dose groups, and the mean plasma concentration was 21.45, 24.60, and 29.68 ng/ml in the 50, 75, and 100 mg group, respectively, 24 h after dose administration; 22.81, 24.57, and 31.41 ng/ml in the 50, 75, and 100 mg group, respectively, 48 h after dose administration, and 12.26, 17.31, and 20.01 ng/ml in the 50, 75, and 100 mg group, respectively, 30 days after dose administration. Overall, 34 patients experienced at least one treatment-emergent adverse event (TEAE). Two patients experienced a serious TEAE and no deaths occurred. There were no extrapyramidal symptoms-related serious TEAEs and no significant changes in any Columbia Suicide Severity Rating Scale parameter were observed during the study. Risperidone ISM provided a sustained release of risperidone that achieved therapeutic plasma levels within the first day. Risperidone ISM was safe, well tolerated, and should be suitable for a 4-weekly administration without oral supplementation.