Background: Hypertension is a common public health problem and often goes unnoticed and can cause damage to the heart and blood vessels which, if untreated, can lead to a stroke or heart attack. However, Hypertension is an iceberg disease in the Africa including Ethiopia. It contributes to the rising burden of cardiovascular disease, premature mortality, reduced quality of life and high costs to the health and social care system and to the economy. Therefore, estimating the prevalence of undiagnosed hypertension and influencing factors with the health seeking behavior is important. Objective: To assess the prevalence of undiagnosed hypertension and associated factors among adults in Hawela Tulla Sub-city, Hawassa, southern Ethiopia in 2019. Method: A community-based cross-sectional survey was conducted among 390 randomly selected adults from selected kebele by multistage sampling technique. Hypertension was defined by SBP≥ 140 mmHg and/or DBP ≥ 90 mmHg. Data was collected by standardized and pre- tested questionnaires and procedures adapted from WHO STEPS survey tools. Data entry, cleaned for completeness and analyzed using SPSS v 25. Descriptive and logistic regression models were used for data analysis. The result declared as statistically significant at p < 0.05. Result: The prevalence of the undiagnosed hypertension among the respondents was 47 (12.3%). The male AOR 3.70, 95% CI (1.64, 8.32) than female contributing. Family history of hypertension had AOR 3.69, 95% CI (1.31, 10.34), being physical inactive AOR 3.21, 95% CI (1.50, 6.84), salty food consumer AOR 3.67, 95% CI (1.26, 10.64), BMI 25 Kg/m2 and more or over weight/obesity AOR 3.06, 95% CI (1.41, 6.65) and not seek health care for some early hypertensive symptoms without serious illness AOR 4.58, 95% CI (1.85, 11.32) when compared to their counterparts, were found to be risk factors for hypertension. Conclusions and Recommendation: The prevalence of undiagnosed hypertension found to be prevalent in the Hawela Tulla Sub-city and calls for intervention. It needs to improve consumption of healthy diet and physical activity. Needs attention to improve health-seeking behavior to prevent undiagnosed hypertension.