Background: Arterial hypertension is one of the leading risk factors for cardiovascular morbidity and mortality. According to the World Health Organization (WHO), approximately 1.4 billion adults aged 30–79 years worldwide were living with hypertension in 2024, of whom only 23% had their blood pressure controlled to the recommended target level. The 2019 national survey in Mongolia reported that 23.6% of adults aged 15–69 years had high blood pressure or were taking antihypertensive medication. Furthermore, the WHO’s 2023 report estimated that only 26% of Mongolian adults aged 30–79 years with hypertension had controlled blood pressure. Nonadherence to antihypertensive medication is an important factor associated with poor blood pressure control. Limited evidence in Mongolia links patients’ understanding of their medications to medication adherence, providing the rationale for this study. Aim: To determine the association between medication understanding and medication adherence among patients with arterial hypertension. Materials and Methods: An analytical cross-sectional study was conducted from July to September 2026 among patients with arterial hypertension attending community pharmacies contracted with health insurance in four districts of Ulaanbaatar: Sukhbaatar, Khan-Uul, Bayanzurkh, and Songinokhairkhan. 165 adults aged 18 years or older with diagnosed hypertension who obtained antihypertensive medications under the health insurance reimbursement scheme were included in the study. The sample size was calculated using Cochran’s formula, assuming a 95% confidence level, an expected proportion of 0.5, a margin of error of 0.08, and an additional 10% allowance for nonparticipation. Data were collected using a structured questionnaire. Medication adherence was assessed with nine questions on a five-point Likert scale. Negatively worded items were reverse-coded, yielding a total score ranging from 9 to 45, with higher scores indicating better adherence. Medication understanding was assessed using six self-report questions, with total scores ranging from 6 to 30. Internal consistency was assessed using Cronbach’s alpha, yielding 0.885 for the medication adherence items and 0.869 for the medication understanding items. Descriptive statistics were used to summarize the data, and the association between medication understanding and adherence was assessed using Spearman’s rank correlation coefficient. The statistical significance level was set at α=0.05, and p-values below 0.05 were considered statistically significant. The study was conducted with approval from the Research Ethics Review Committee of the Mongolian National University of Medical Sciences, dated June 26, 2026 (approval No. 2026/06-01). Voluntary informed consent was obtained from all participants, and confidentiality of the collected information was maintained. Results: Of the 165 participants, 118 (71.5%) were female, and 47 (28.5%) were male. A total of 99 participants (60.0%) reported having comorbid diagnoses. The mean total medication adherence score was 30.87±7.37, while the mean medication understanding score was 21.99±4.02. Among the six medication understanding items, understanding the need for continuous medication use had the highest mean score (3.90±0.76), while understanding medication-related adverse effects had the lowest (3.28±0.95). Among the nine medication adherence items, the item related to discontinuing medication when blood pressure returned to normal had the lowest mean score (3.12±1.30). Medication understanding was weakly and significantly positively correlated with medication adherence scores (Spearman’s ρ=0.320; p<0.001). In multivariable analysis, each one-point increase in medication understanding score was associated with an average 0.404-point higher medication adherence score (B=0.404; 95% confidence interval [CI]: 0.135–0.672; p=0.003). Compared with participants aged 18–34 years, those aged 65 years or older had significantly higher adherence scores (B=5.814; p=0.001). Compared with participants who reported that their blood pressure was within the normal range, those who reported that their blood pressure remained elevated had lower medication adherence scores (B=−3.331; p=0.020). Participants who were unaware of their blood pressure status also had lower adherence scores (B=−4.016; p=0.013). Conclusion Medication understanding was weakly but significantly positively associated with medication adherence among patients with arterial hypertension.