The study aimed to estimate the prevalence and associated factors of undiagnosed hypertension (HTN) among adults in the Central African Republic (CAR). In the cross-sectional 2017 CAR (Bangui and Ombella M'Poko) STEPS survey, 3265 persons aged 25 to 64 years (non-pregnant and with complete blood pressure measurement), responded to an interview, biomedical and physical, including blood pressure, measurements. Undiagnosed HTN was classified as systolic BP ≥ 140 mmHg and/or diastolic BP ≥ 90 mmHg among adults who had never been told by a doctor or other health worker that they had raised blood pressure or hypertension and had not been taking antihypertensive medication. Binary logistic regressions are used to estimate factors associated with undiagnosed HTN. Among those with HTN (N = 1373), the proportion of undiagnosed HTN was 69.8% and 30.2% diagnosed HTN. In the adjusted logistic regression analysis, male sex (AOR: 2.12, 95% CI 1.39–3.23), current tobacco use (AOR: 1.58, 95% CI 1.03–2.42), and high physical activity (AOR: 1.93, 95% CI 1.00–3.71) were positively associated, and age (AOR: 0.75, 95% CI 0.59–0.96), and underweight (AOR: 0.58, 95% CI 0.37–0.90) were inversely associated with undiagnosed HTN. In addition, among men, ever screened for glucose (AOR: 0.07, 95% CI 0.02–0.27) was negatively associated with undiagnosed HTN, and among women, married or cohabiting (AOR: 1.20, 95% CI 1.00–1.44), current heavy drinking (AOR: 1.41, 95% CI 1.04–1.91) were positively associated with undiagnosed HTN. Seven in ten of the adult population with HTN had undiagnosed HTN in CAR. Efforts should be reinforced to screen for HTN in the general population.