OBJECTIVES: In recent years, research on behavioral and psychiatric problems of adults with Prader-Willi syndrome (PWS) has gained attention. However, no report is available regarding the relationship between psychiatric illness and type 2 diabetes mellitus (T2DM) in patients with PWS. Therefore, we evaluated a behavioral assessment to address the lack of data on the association between psychiatric behavior and T2DM. METHODS: This was a retrospective single-center study of patients with PWS. Patients with PWS whose blood tests were performed in our hospital between January 2018 and December 2019 and aged >10 years were included. We evaluated the data, including the behavioral patterns of Japanese PWS patients with T2DM. RESULTS: Overall, 114 patients were evaluated; 33 patients (28.9%) developed T2DM. The age of T2DM onset was 18.0 years (interquartile range [IQR], 14.6-21.4 years). The median body mass index at T2DM onset was 33.7 kg/m2 (IQR, 30.0-37.4 kg/m2). Between-group comparisons of the intelligence quotient, Food-Related Problem Questionnaire (FRPQ), and Japanese versions of the Short Sensory Profile and Aberrant Behavior Checklist showed a significant difference only in FRPQ scores (p=0.003). CONCLUSIONS: The occurrence of T2DM among Japanese patients with PWS remains high. Only the FRPQ was significantly different between the T2DM and the non-T2DM group.