Background. Though onchocerciasis control and elimination through community directed treatment with ivermectin were conducted for the last fifteen years, prevalence of onchocerciasis and factors associated with it in the study area are yet not known. The aim of the current study is to assess prevalence of onchocerciasis and associated factors among adults aged greater than or equal to fifteen years in Semen Bench district, Bench Maji zone, southwest Ethiopia: community based cross-sectional study 2018. Methods. Community based cross-sectional study was conducted on 553 study participants selected by multistage sampling in April 2018. Data were collected using a pretested interviewer-administered questionnaire. Data were entered using EpiData version 3.1 and exported to SPSS version 20 for statistical analysis. Descriptive statistics were done to summarize dependent and independent variables. Bivariate logistic regression was done to select candidate variables. Multivariable logistic regression was performed to identify independent predictors of onchocerciasis infection. Adjusted odds ratios with 95% CI were calculated to assess association and statistical significance, respectively. Confidence interval was used to declare statistical significance. Result. The overall prevalence of onchocerciasis infection in the study area was 6.32%. Age category of 35-44 years (AOR: 13.48, 95%CI: 3.51, 51.76), age of 45 years and above (AOR: 9.41, 95% CI: 2.26, 39.06), male sex (AOR 4.568, 95% C.I: 1.622, 12.861), not being compliant with ivermectin treatment (AOR: 3.804, 95%CI: 1.524, 9.49), and residing at less than 2Km from the river (AOR: 9.15, 95%CI: 3.9, 21.49) were significantly associated with onchocerciasis infection. Conclusion and Recommendation. After more than a decade of treatment with ivermectin, onchocerciasis in the study area is still hypoendemic. Zonal health department and other stakeholders should evaluate therapeutic coverage and community directed treatment with ivermectin in the study area. Zonal health department with other stokeholds should give community based information education communication, giving due attention to older ages, male residents, and those living near the rivers. Further community based study should also be done to identify factors hindering the community compliance with the treatment.