Introduction: Hypertension, including poorly controlled blood pressure, is the major global health problem that affects one billion people worldwide. Limited studies have been conducted on prevalence of uncontrolled hypertension and associated factors among adult hypertensive patients in Ethiopia. Objective: The aim of this study was to determine the prevalence of uncontrolled hypertension and associated factors among adult hypertensive patients at Jimma University Teaching and Specialized Hospital. Methods: Institution-based cross-sectional study was conducted at the chronic illness clinic of Jimma University Specialized and Teaching hospital from March 09 to April 13, 2016. A total of 345 hypertensive patients were selected using systematic sampling technique. Data were collected using structured questionnaire through face-to-face exit interview and chart review. Data were analyzed using Statistical Package for the Social Sciences (SPSS) version 20.0 software. The bivariate and multivariable analysis was done to identify factors of uncontrolled hypertension. Results: More than half, 52.7%, of the patients had uncontrolled hypertension. Lack of awareness of hypertension-related complications (adjusted odds ratio [AOR]=2.140, 95% confidence interval [CI]=1.272-3.600, p=0.004), nonadherent to smoking abstinence (AOR=3.935, 95% CI=1. 065-14.535, p=0.004), nonadherent to alcohol abstinence (AOR=2.477, 95% CI=1.074-5.711, p=033), Khat (Catha edulis) chewing (AOR=2.518, 95% CI=1.250-5.073, p=0.010), overweight (AOR=2.241, 95% CI=1.239-4.053, p=0.008), middle age (AOR=7.893, 95% CI=1. p=0.008), and old age (AOR=9.944, p=0.001 were significant predictors of uncontrolled hypertension. Conclusion: The prevalence of uncontrolled hypertension was high at Jimma University Teaching and Specialized hospital among patients with hypertension. Unhealthy lifestyles were major factors. Continuous health education on lifestyle practices and hypertension-related complications in each follow-up visit through nurses, physicians, and pharmacists are very essential to avert the problem.