Background Hypertension is a major risk factor for cardiovascular disease, which is the leading cause of mortality globally. Patient’s adherence to treatment is a cornerstone factor in controlling hypertension and its complications. This study assesses hypertension patients’ adherence to treatment and its associated factors. Methods This cross-sectional study conducted in Riyadh, Saudi Arabia. The study targeted outpatients aged ≥18 years who were diagnosed with hypertension. Participants were recruited using a systemic sampling technique. The two main measurements were assessing adherence rate of antihypertensive medications using Morisky scale and identifying predictors of poor medication adherence among hypertensive patients including socio-economic and demographic data, health status, clinic visits, medication side effects, medications availability, and knowledge. Descriptive and logistic regression analyses were performed to assess factors associated with poor adherence. Results A total of 306 hypertensive outpatients participated in this study. 42.2% of participants were adherent to antihypertensive medications. Almost half of participants (49%) who reported having no comorbidities were adherent to antihypertensive medications compared to participants with one or more than one comorbidities 41, 39% respectively. The presence of comorbid conditions and being on multiple medications were significantly associated with medication adherence (P-values, respectively, < 0.004, < 0.009). Patients with good knowledge about the disease and its complications were seven times more likely to have good adherence to medication (P <  0.001). Conclusions Non-adherence to medications is prevalent among a proportion of hypertensive patients which urges continuous monitoring to medication adherence with special attention to at risks groups of patients. Patients with comorbidities and on multiple medications were at high risk of medication non-adherence. Patients’ knowledge on the disease was one of the main associated factors with non-adherence.