Background: Hypertension is a common health problem estimated to affect around one billion people worldwide. Over the last 20 years, there has been a steady increase in the prevalence of hypertension in Sultanate of Oman. Nonadherence to medications is a common issue across patients with hypertension. Studies showed that sixty percent of hypertensive patients discontinue their antihypertensive drugs after only one year. Non-adherence to medications has serious consequences in patient's health and in the cost of health services. Aim: The aim of this study is to assess adherence to antihypertensive drugs by 8 item Morisky Medication Adherence Scale (MMAS-8) and Medication Possession Ratio (MPR) in patients attending adult outpatient clinic at Sultan Qaboos University Hospital (SQUH). Methods: This is a cross sectional study conducted in multiple specialized outpatient clinics at SQUH. The inclusion criteria for the study were; hypertensive patients aged above 18 years and taking at least one antihypertensive drug. Two hundred and sixty five hypertensive patients who agreed to participate in the study received a copy of MMAS-8. Besides that, adherence using MPR was measured by recording prescriptions which contained at least one antihypertensive drug and their date of prescribing for a 12 month period. Further data was collected such as concomitant diseases; blood pressure and treatment related variables. The collected variables were tested for association with MMAS-8 and MPR. Pattern of prescribing antihypertensive drug was also evaluated. Data was analyzed by using SPSS version 20. Results: Adherence using MMAS-8 for low, medium and high were 11%, 46% and 42%, respectively. MPR results revealed that high adherence was dominant with (71%) followed by medium adherence (18%) and low adherence with (11%). Adherence based on MMAS-8 increased as the age and number of comorbidities increased. However, no statistical significance was found. Males were more likely to be more adherent than females with statistical significance found in MPR (p<0.05). Unlike MMAS-8, only MPR was significantly associated with the level of blood pressure control (p<0.05). The most common three concomitant diseases were dyslipidemia (45%), ischemic heart disease (41%) and diabetes mellitus (40%). The top four prescribed antihypertensive agents were diuretics, angiotensin receptor blocker, beta blockers and calcium channel blockers. Concordance which is the degree of agreement between methods was measured and results revealed concordance between low or medium adherence was 9% while high adherence was 51%, making the overall concordance of 60%. Conclusion: Assessment of adherence to medications is an important aspect in designing future therapeutic plan in terms of adding or replacing medications or even going to further investigations for diagnosing purposes. Overall, there was a non-adherence to antihypertensive drugs. However, MPR tend to overestimate adherence. The two methods could not be used as gold standard method to assess adherence and more research is needed to ensure validity of these methods and compare them with other methods like direct assessment of medication adherence.