Background: Diabetes mellitus is recognized as one of the emerging public health problems in developing countries. Evidence suggests that half of the Diabetic patients are undiagnosed. Undiagnosed Diabetes Mellitus impose substantial implications because subjects remain untreated and at risk for complications. Objective: To assess the prevalence of undiagnosed Diabetes Mellitus and its associated factors among adults aged 18 years and above in Bahir Dar City, Northwest Ethiopia. Methods: A community based cross-sectional study was conducted on 456 adults in Bahir Dar city from February 20 to March 20 /2017. Out of 9 sub-cities, three sub-cities were selected using a simple random sampling technique by lottery method, and systematic random sampling techniques was used to select household. Finally, from those households, a family member who meet all eligibility requirements was selected from adults who were present during the data collection. While the prevalence of Undiagnosed DM was evaluated by measuring the random blood sugar level, associated factors were determined through physical measurement and structured questionnaire, by adopting the WHO stepwise approach. Data analysis was done using SPSS version 20.0. Software. Bivariate and multivariable logistic regression analysis was used to infer casual relationships between the independent and dependent variables. Results: The prevalence of undiagnosed diabetes mellitus was 5.9% (95%CI: 3.7%- 8.3%). Overweight and obesity of the participants (AOR= 5.40, 95%CI: 2.22- 13.15), individuals with in the age group >35 years (AOR=4.24, 95%CI: 1.15- 15.55) and marital status (widowed, divorce and separate) (AOR= 0.27, 95%CI: 0.11- 0.64) were found to have statistically significant association with DM prevalence. Conclusion: The current prevalence of undiagnosed diabetes mellitus in Bahir Dar city was high compared to estimated national prevalence of Ethiopia, 4.85% reported by the IDF in 2014 and it was found to have association with age, marital status (widowed, divorce and separate), overweight and obesity. Recommendation: Urban health extension programs shall target households to implement dietary and physical activity practice that help reduce weight