Objective Medication adherence is a major challenge in the treatment of older patients; however, they are under‐represented in research. We undertook a systematic review focused on older patients to assess the reasons underlying non‐adherence in this population. Methods We searched multiple electronic databases for studies reporting reasons for non‐adherence to medication regimens in patients aged 75 years and over. Our results were not limited to specific diseases, health‐care settings, or geographical locations. The quality of eligible studies was assessed using the Newcastle‐Ottawa Scale. A narrative synthesis of findings was performed. Results A total of 25 publications were included, all of which were in community settings. Frequent medication review and knowledge regarding the purpose of the medication were positively associated with adherence. Factors associated with poor adherence were multimorbidity, cognitive impairment, complex regimens with multiple prescribing physicians, and problems with drug storage or formulation. Conclusion These findings suggest that interventions to improve adherence could focus on medication review aimed at simplifying regimens and educating patients about their treatment. Groups with poor adherence that may benefit most from such a model include patients with multiple comorbidities and cognitive impairment.