Background:Iron/folic acid supplementation is feasible and cost effective strategy to control and prevent anemia in pregnancy. In Ethiopia,the national data suggests that from all pregnant women supplemented with these tablets only 0.4% consumed more than 90 tablets during their pregnancy time. The factors for this low adherence are not clearly known. Objective:To assess adherence to Iron/folic acid supplementation and associated factors among pregnant women attending antenatal care in publichealth facilities of AssosaZone Administration, Western Ethiopia from December toMarch2019. Method: Health institution basedcross sectional study was conducted using quantitative methodson 395 pregnant women attending antenatal care clinic in fourpublic health facilities. Systematic sampling method was used.An interview with pre tested structured questionnaire was carried out.Data were enteredand exported to Statistical Package for Social Science version 20.0 for analysis. Bivariate and multivariable Logistic regression with odds ratios along with the 95% confidence interval were computed and interpreted accordingly.P-value <0.05 wasdeclared as statistically significant. TheEthical clearance was obtained from Bahir DarUniversity Institute of Technology School of Research and Postgraduates Studies. Result: The adherence rate was 55.5%(95%CI, 50.5%-60.4%). Factors significantly associated with adherence to Iron/folic acid supplementation were maternal educational status (AOR=4.0, 95% CI=1.88 - 8.54), early registration for Antenatal care (AOR= 1.8, 95% CI=1.06 – 3.11), knowledge of Iron/folic acid supplementation (AOR= 2.1, 95% CI= 1.24 - 3.56), and history of anemia during current pregnancy (AOR= 7.9, 95% CI=4.4414.01).Forgetfulness and fear of side effects were the major reasons for missing the doses of tablets. Conclusion and Recommendations:Adherence rate is low. Increasing knowledge of women about the supplementation through adequate counseling, community education and media are recommended to increase adherence.