Background: Hypertension is the leading cause of morbidity and mortality among noncommunicable diseases. The rate of blood pressure (BP) control among hypertensive patients is poor and the reasons for poor control of BP remain poorly understood globally. Therefore, this study aimed to assess the magnitude and determinants of uncontrolled blood pressure among adult hypertensive patients on follow-up at Nekemte referral hospital (NRH). Methods: A hospital-based cross-sectional study was conducted from February to April 2018 at NRH. BP control status was determined by the average consecutive BP recordings across the 3 months. The data was entered and analyzed using SPSS version 20.0 and p-value <0.05 was considered statistically significant. Results: Out of 297 study participants included, the majority were females, 181 (60.9%), and the mean age of the patients was 59.4 ±10.4 years. About half, 137 (46.12%), of the patients had at least one comorbidity and the most common class of anti-hypertensive medication was angiotensin-converting enzyme inhibitors (88.2%). The mean of systolic blood pressure was 132.41± 15.61mmHg, while the mean of diastolic blood pressure was 84.37± 9.32 mmHg. The proportion of participants with optimally controlled BP was 63.6% and 36% were adherent to their medications. Male sex (Adjusted Odd Ratio [AOR]: 1.89, 95% CI: 1.09-4.84), illiteracy (AOR= 1.56, 95% CI: 1.22-6.78), duration of hypertension diagnosis > 10 years (AOR= 2.01, 95% CI: 1.04-16.11), non-adherence (AOR= 3.14, 95% CI: 1.35-10.76) and lack of physical exercise (AOR= 2.8, 95% CI: 1.16-6.74) were positively associated with uncontrolled BP status. Whereas age older than 55 years (AOR= 0.38, 95% CI: 0.11-0.92) was negatively associated with uncontrolled BP. Conclusion: BP control was relatively achieved in about two-third of pharmacologically treated patients. We recommend better health education and care of patients to improve the rate of BP control status.