Background: Notwithstanding the availability of effective treatments, asymptomatic nature and the interminable treatment length, adherence to medication remains a substantial challenge among patients with hypertension. Suboptimal adherence to BP-lowering agents is a growing global concern that is associated with the substantial worsening of disease, increased service utilization and healthcare cost escalation. This study aimed to explore medication adherence and its associated factors among hypertension outpatients attending a tertiary-level cardiovascular hospital in Tanzania. Methods: The pill count adherence ratio (PCAR) was used to compute adherence rate. In descriptive analyses, adherence was dichotomized and consumption of less than 80% of the prescribed medications was used to denote poor adherence. Logistic regression analyses was used to determine factors associated with adherence. Results: A total of 849 outpatients taking antihypertensive drugs for ≥1 month prior to recruitment were randomly enrolled in this study. The mean age was 59.9 years and about two-thirds were females. Overall, a total of 653 (76.9%) participants had good adherence and 367 (43.2%) had their blood pressure controlled. Multivariate logistic regression analysis showed; lack of a health insurance (OR 0.5, 95% CI 0.3-0.7, p<0.01), last BP measurement >1 week (OR 0.6, 95% CI 0.4-0.8, p<0.01), last clinic attendance >1 month (OR 0.4, 95% CI 0.3-0.6, p<0.001), frequent unavailability of drugs (OR 0.6, 95% CI 0.3-0.9, p = 0.03), running out of medication before the next appointment (OR 0.6, 95% CI 0.4-0.9, p = 0.01) and stopping medications when asymptomatic (OR 0.6, 95% CI 0.4-0.8, p<0.001) to be independent associated factors for poor adherence. Conclusion: A substantial proportion of hypertensive outpatients in this tertiary-level setting had good medication adherence. Nonetheless, observed suboptimal blood pressure control regardless of a fairly satisfactory adherence rate suggests that lifestyle modification plays a central role in hypertension management.