Background: Common mental disorders such as depressive and anxiety disorders are a leading cause of disability and major public health issue in globally. However, the magnitude and associated factors at the community level is not well investigated in Ethiopia. Objectives: To assess prevalence and associated factors of common mental disorders among adult in Bahir Dar City administration. Method: A community based cross-sectional study was conducted on a randomly selected of 665 adults from October 10 to October 27, 2019. Self-reporting questionnaire (SRQ-20) was used to determine the prevalence of common mental disorders. Common mental disorders were defined as a cut-off point ≥ 8 score from SRQ-20 questionnaire. Data were collected with interviewer administer structured questionnaire by trained data collectors. The data were entered and analyzed by using SPSS version 23. Both simple and multiple binary logistic regression analysis were used to identify factors associated with common mental disorders. Adjusted odds ratio with 95% confidence level was used to determined significant association with outcome variables. Result: The prevalence of common mental disorders in the study area was 34.1 %( 95% CI=30%-38%). In this study, being females (AOR=1.78; 95% CI 1.12-2.84), divorced (AOR=2.45; 95% CI 1.37-4.37), widowed (AOR=2.65; 95% CI 1.44-4.86), unable to read and write (AOR=2.93; 95% CI 1.01-8.46), family history of mental illness (AOR=3.61;95% CI 2.27-5.75),chronic illness (AOR=3.84; 95% CI 2.38-6.20), alcohol users (AOR=3.29; 95% CI 1.86-5.81) and history of accident (AOR=5.06; 95% CI 2.43-10.51) were independently associated with common mental disorders. Conclusion: The result of current study revealed a high prevalence of common mental disorders. Being female, widowed, divorced, unable to read and write, alcohol users, family history of mental illness, presence of chronic illness and history of accident are risk factors for development of common mental disorders. Ministry of health should design multi-sectorial approach. Keywords: Prevalence, Associated factors, Common mental disorders, Adult, Ethiopia