Introduction: Globally, hypertension is an important risk factor for cardiovascular diseases and all-cause mortality. Despite this high public health burden, no community-based evidence regarding the magnitude and related factors of hypertension has been reported in the study area. Objective: The aim of this study was to determine the prevalence and associated factors of undiagnosed hypertension in Debre Markos town, North-West Ethiopia, 2020. Method: A community-based cross- sectional study design was conducted from January to March 2020. Socio-demographic and behavioral characteristics of the participants were collected using interviewer-administered semi-structured questionnaires. Physical measurements including body mass index, waist circumference, and blood pressure were collected using standardized measuring instruments. Statistical analysis was collected using SPSS version 21 software. Univariate logistic regression model was used to present the results. The p value less than 0.05 in a multivariable analysis was considered statistically significant at 95% confidence interval. Results: From a total of 630 samples, 600 participants, 270 (45%) males and 330 (55%) females with a mean age of 36.32 ± 12.48 years, were participated giving a response rate of 95.2%. The prevalence of undiagnosed hypertension among adults was 12.7%. Of these, 5.5% were males and 7.2% were females. Factors including age (above 60 years) (adjusted odds ratio = 2.92, 95% confidence interval (1.05, 8.06), p = 0.03), body mass index (overweight and obese) (adjusted odds ratio = 3.85, 95% confidence interval (2.16, 6.87), p < 0.01), fatty diet consumption (adjusted odds ratio = 1.88, 95% confidence interval (1.03, 3.42), p = 0.04), family history of hypertension (adjusted odds ratio = 2.21, 95% confidence interval (1.04, 4.69), p = 0.04) were significantly associated with hypertension. Conclusion: The prevalence of undiagnosed hypertension among adults was found to be remarkable. Therefore, a community-based health education and hypertension screening strategies are needed to prevent the problem early in life in Debre Markos town and the nation at large.