Great efforts to advance the diagnosis and treatment of hypertension for controlling hypertension have been made; however, the rates of uncontrolled blood pressure are still high. This study explored the rate of uncontrolled hypertension in patients with hypertension managed in an urban hospital of Vietnam and identified associated factors. A cross-sectional survey was performed from August to October 2019 among hypertensive patients at an urban hospital in Hanoi, Vietnam. Blood pressure was evaluated at the time of medical examination. Demographic, clinical, and behavioral characteristics were also collected. Multivariate logistic regression was used to identify the factors related to uncontrolled hypertension. Among 220 patients, the rate of uncontrolled hypertension was 40.5%. Females had a lower likelihood of having uncontrolled hypertension compared to males (adjusted OR = 0.33; 95% CI = 0.11–0.98). Higher duration of diseases (adjusted OR = 1.07; 95% CI = 1.01–1.14) and higher body mass index (adjusted OR = 1.23; 95% CI = 1.05–1.45) were positively associated with uncontrolled hypertension. Patients who carried supplies needed for self-care, cut down on stress, exercised regularly, and stopped/cut down on smoking were also less likely to develop uncontrolled hypertension. This study reveals that uncontrolled hypertension was common among hypertensive patients in Vietnam. Improving self-care capacity and encouraging healthy behaviors are critically important to control blood pressure, particularly among patients who were males and had high disease duration and body mass index.