The current study was conducted to determine the impact of health literacy and factors related to adherence to drug treatment, using the model proposed by the World Health Organization, in older adults with hypertension residing in informal settlements in Hamadan. This cross-sectional study was conducted on 405 patients in Hamadan city, located in the western part of Iran. Data were collected using an interviewer-administered questionnaire that included the 5-dimensional model proposed by the World Health Organization, Health Literacy for Iranian Adults, and Morisky Medication Adherence Scale-8. A two-stage sampling procedure was used to select patients from 14 comprehensive health service centers and health bases. The data were analyzed using SPSS v.24. The study found that medication adherence was suboptimal in 63% of the participants. Additionally, 87.5% of patients had inadequate or insufficient health literacy. Factors related to medication adherence included age (odds ratio (OR) = 1.07), annual income (OR = 0.17), duration of hypertension (OR = 7.33), health literacy (OR = 1.03), self-reported health status (P < 0.05), and regular medication use (P < 0.008). The results of this study indicate that more than half of the older adults in the study had suboptimal medication adherence and insufficient health literacy. The study also found that various factors, such as socioeconomic status, disease and treatment-related factors, and patient-related factors, influence medication adherence among older adults.