Background Medication regimen complexity includes various aspects of a regimen, including the dosage form, number of medications and need for additional information for use. Complicated medication regimens cause non-adherence to prescribed medications, making it essential to evaluate medication regimen complexity in older adults to improve adherence to prescribed medications and clinical outcomes. The medication regimen complexity index is currently the most widely used scale for quantifying regimen complexity; however, it has yet to be adopted in Japan. Objective This study aimed to translate the medication regimen complexity index to Japanese and assess its reliability and validity for application in elderly patients in Japan. Setting This study was conducted in a clinic that provides home medical care to patients in the southern part of Ibaraki prefecture, Japan. Method The validation process consisted of translation of the original English version of the medication regimen complexity index to Japanese followed by back-translation to English, comparison of the back-translated and original versions, pilot testing, and assessment of the Japanese version by two raters using the med