The purpose of this study was to identify factors that influence medication adherence in older patients as assessed by nurses in an acute care hospital in Japan. A total of 3271 nurses with at least 3 years of clinical experience working in an acute care hospital were included in the study. A questionnaire survey was conducted on one case of an older patient for whom the nurses were currently providing medication self-management support. 629 responses were received. The total medication adherence scores of older patients were found to be associated with age, use of neuropsychiatric medications, and caregiver intervention. The subfactors of medication adherence had different influencing factors and were associated with factors such as age, use of neuropsychiatric medications, pharmacist intervention, number of types of medications, and previous diseases.