Background: Hypertension is defined as two or more readings of systolic blood pressure measurement of 130 mmHg or higher or diastolic blood pressure measurement of 80 mmHg or higher. In the early stages, symptoms of hypertension (HTN) are often not detectable and thus many people with the disease are left undiagnosed. Undiagnosed HTN increases the risk of complications such as renal failure, myocardial infarction, heart failure, stroke and premature death. There is paucity of data concerning undiagnosed hypertension in southern Ethiopia. This study is aimed to assess prevalence of undiagnosed hypertension and its associated factors among adult peoples.Method: Community based Cross-sectional study design was conducted. English version questionnaire was used to collect data from 574 households. The data were collected and analyzed by SPSS version.23. The association between "blood pressure and socio-demographic characteristics', life style factors, behavioral factors..." was tested by using multivariate logistic regression at P-value less than 0.05 and 95% CI.Result: Out of 574 participants, 108 (18.8%) of the study participants were current cigarette smokers and 143 (24.9%) were khat chewers. A sedentary lifestyle was seen in 106 (18.5%) of the studied people. Undiagnosed HTN was observed in 47 (15.3%) of the study participants. A total of 260 (45.3%) of respondents were identified as having undiagnosed hypertension. Body mass index (BMI), sedentary life style, cigarettes smoking, health seeking behavior, dietary habit and monthly income were significant predictors of undiagnosed HTN. Conclusion:Almost 50% of the adult population in the southern of Ethiopia shows undiagnosed hypertension and this is significantly correlated with body mass index (BMI), sedentary life style, cigarettes smoking, health seeking behavior, dietary habit and monthly income.