1-(hydroxymethyl)-5,5-dimethylimidazolidine-2,4-dione (MDM hydantoin) is a commonly used antiseptic preservative in cosmetics. However, limited toxicity information data are available for this chemical. The aim of this study was to obtain toxicity data for MDM hydantoin through single- and repeated-dose toxicity studies in Sprague–Dawley (SD) rats. In the single-dose toxicity study, MDM hydantoin was administered once orally to SD rats at four doses (5, 50, 300, and 2000 mg/kg/day). There was no significant difference in mortality, clinical signs, and body weight change for 14 days among the animals treated with the different doses in this study. Hence, the approximate lethal dose of MDM hydantoin was considered higher than 2000 mg/kg/day. Based on the results of the dose-range finding study, a 28-day repeated-dose oral toxicity study was conducted. MDM hydantoin was administered orally to SD rats at doses of 125, 250, 500, and 1000 mg/kg/day throughout an experimental period of 28 days. In the repeated-dose oral toxicity study, the adverse effects caused by MDM hydantoin were not detected in terms of body weight, clinical signs, food and water intake, hematology, organ weights, gross pathology, and histopathology. Therefore, the no-observed-adverse-effect level of MDM hydantoin was considered to be greater than 1000 mg/kg/day.