Background: Blood transfusion saves millions of lives each year globally. But, it was associated with certain risks which can lead to adverse consequences. However, there is paucity of information regarding to the sero-prevalence and risk factor of transfusion-transmittable infections among blood donors in Eastern Ethiopia. Objective: The aim of this study was to determine the sero-prevalence of transfusion-transmittable infections and associated factors among blood donors in Eastern Ethiopia from February to March 2018. Methods: An institutional-based cross-sectional study was conducted among 500 blood donors in Eastern Ethiopia. Data were collected using a structured questionnaire and laboratory blood screening. Data were analyzed using Statistical Package for Social Sciences version 20. Statistically significance was considered at p < 0.05. Results: The overall sero-prevalence of transfusion-transmittable infections was 12.4% (95% confidence interval: 9.5, 15.3). Hepatitis B virus (6.6%) and syphilis (3.4%) were found at high magnitude. Those with family having human immunodeficiency virus or hepatitis (adjusted odd ratio = 2.91; 95% confidence interval: 1.33, 6.33), giving care for human immunodeficiency virus or hepatitis patient (adjusted odd ratio = 3.24; 95% confidence interval: 1.49, 7.07), multiple sex partner (adjusted odd ratio = 2.56; 95% confidence interval: 1.21, 5.19), unsafe sex (adjusted odd ratio = 2.99; 95% confidence interval: 1.51, 5.92), dental procedure (adjusted odd ratio = 2.75; 95% confidence interval: 1.20, 6.28), and had no formal education (adjusted odd ratio = 2.46; 95% confidence interval: 1.24, 4.86) were significantly associated with transfusion-transmittable infections. Conclusion: The prevalence of transfusion-transmittable infections in this study was relatively high compared to studies conducted earlier in Ethiopia. Factors such as unsafe sex, multiple sex partner, dental procedure, and family with human immunodeficiency virus or hepatitis, provide health care for human immunodeficiency virus–infected or hepatitis patients, donor type, and educational level were significantly associated with transmission-transmittable infections. Thus, health promotion about prevention and control transmission-transmittable infections should be given to the community by considering the identified risk factors. Each of the blood banks and regional health bureaus in the study area should mobilize community for increasing repeated voluntary donors through promotion of blood bank activity.