Medication adherence is a major component in the treatment of chronic patients, especially the older adults. It acts as an essential part of a successful therapeutic outcome, increasing treatment benefits, and decreasing hospital admissions, health care utilization, and cost. Limited research regarding medication adherence has been conducted nationwide; however, data among the older adults is sparse due to the lack of research on this topic. Therefore, this study aimed to determine the prevalence of medication nonadherence among older adults' Saudi patients with chronic diseases living in Al-Madinah Al-Munawwarah city. A cross-sectional study was conducted on outpatient older patients with chronic diseases who visited clinics at different hospitals in Al-Madinah Al-Munawwarah city, from March until December 2022. The questionnaire was published in a conversational Arabic dialect. The adherence level was measured by the Adherence to Refills and Medications Scale (ARMS) and the reasons for medication nonadherence were collected by using the Cluig scale. 65.1% of females and 34.9% of males have participated in this study. According to the ARMS scale, 67.9% of patients had poor medication adherence and 32.1% had good medication adherence. Poor adherence was higher in females compared to males, with a p value = 0.018. For educational level, the ARMS score in graduates was higher than patients who finished middle school and illiterate patients. Based on the Cluig scale, the most common reasons for medication nonadherence were forgetfulness, feeling well, polypharmacy, and subsequent avoidance of side effects. The findings indicate the gender-based differences in the factors that associated with medication nonadherence among older adults. Medication adherence is crucial especially among older adults to improve clinical outcomes. The current results highlight the prevalence of medication adherence among the older adult population. This result shows the gender-based differences in the factors associated with medication nonadherence among the older adult population.