<p>This study aimed to develop a Japanese version of the Multimorbidity Treatment Burden Questionnaire (J-MTBQ) and examine its validity and reliability. The MTBQ was translated into Japanese using forward and backward translation and cognitive interviews assessed content validity in a Japanese context. We conducted a nationwide postal survey using a representative panel of the Japanese adult population. Eligible participants in this study were patients with multimorbidity (≥ 2 long-term conditions). Structural validity, convergent validity, known-groups validity, and internal consistency reliability were assessed. Cognitive interviews found good acceptability and content validity. The survey was completed by 383 patients. Confirmatory factor analysis supported the scale’s structural validity and a one-factor solution. In hypothesis testing, the J-MTBQ score had a negative association with health-related quality of life and self-rated health; and a positive association with the number of long-term conditions and prescribed medications. Cronbach’s alpha indicated a high level of internal consistency reliability. We have developed a J-MTBQ and examined its validity and reliability. This scale can be used to identify patients experiencing high treatment burden, to improve the quality of care for patients with multimorbidity, and to assess the impact of interventions for multimorbidity in Japanese-speaking adults.</p>