Objective: To develop a medication-adherence assessment tool for nurses to acquire greater understanding of autonomous medication self-management by older adults. Methods: This study was based on an exploratory sequential study design using a qualitative inductive study and pretest results from which we created a draft medication-adherence assessment tool containing 74 items. Then, to verify reliability and validity, questionnaires were sent to nurses with three or more years of clinical experience in randomly selected acute care hospitals or in home-visit nursing stations. The research was conducted with the approval of the Shimane University Nursing Research Ethics Review Committee. Results: From a total of 747 responses and following an item analysis, 40 items were selected and exploratory factor analysis was performed. From this, we prepared a "medication-adherence assessment tool for older adults" consisting of 40 items and 6 factors. The six factors were the following: 1) active participation in their treatment, 2) stable medication compliance, 3) medication behavior and stable lifestyle, 4) health control ability with continuous medication, 5) inhibitors of medication self-management, and 6) capability of medication record management. This assessment tool was proven to show internal consistency based on Cronbach's α coefficient, criterion-related validity, and constructive concept validity. Conclusions: The "medicationadherence assessment tool for older adults" allows nurses to systematically assess the status of autonomous selfmanagement of medication by older adults from a medication-adherence perspective.