Background: Common mental disorders such as depression, anxiety and somatic symptoms are a public health concern. It has physical health, psychological and economic consequences. These illness is an emerging issue in Ethiopia.Therefore, this study aimed to assess the prevalence of common mental disorders and associated factors among residents of south Gondar zone 2018.Methods: In this cross-sectional study,731 respondents were recruited for interrviews using a systematic random sampling technique.The self reporting questionnaire was used to assess common mental disorders. List of Threatening Experiences, and the Oslo social support instruments were used to assess the factors. We computed bivariate and multivariable binary logistic regressions to assess factors associated with common mental disorders. Statistical significance was declared at P-value <0.05.Result: A total of 731 participants were interviewed, with a response rate of 98.5%. The prevalence of common mental disorders was 29.7% with (95% CI: 26.4-33.1).In the multivariate logistic regression,female sex (adjusted odds ratio (AOR)= 2.47, 95% CI : 1.68, 3.62),poor social support( AOR =2.34, 95% CI:1.50, 3.64), family history of mental illness(AOR =2.15, 95% CI:1.32-3.51), current use of khat(AOR =1.69, 95% CI: 1 .07, 2.64) and tobacco(AOR=1.71, 95% CI:1.04-2.84), and rural residence(AOR=2.01, 95% CI: 1.35, 3.01) were factors significantly associated with CMDs.Conclusion and recommendations: The prevalence of common mental disorders was found to be high. Female sex, current substance use (khat chewing (leaves) and tobacco smoking), unemployment, rural residence, family history mental illness, and poor social support were significantly associated with common mental disorder. Therefore; It is necessary to give emphasis to individuals with family history of mental illness, women, and history of mental illness.